I recently came across some archival footage which gives some glimpses of some quite rare US medium tanks developed in the 1920s. The footage features the M1921, the T2 Medium Tank and a Christie Tank.
The US tank arm subsequently abandoned the various medium tank designs they’d been working on and shifted towards cheaper light tanks. Always special finding archival footage, hope you enjoy the video.
The need for a lightweight self-propelled anti-tank gun was identified in the late 1940s. The T101 development program took just over 5 years and $2.5 million dollars to develop what became the M56.
The M56 was intended to act as an airmobile support weapon for the US Army’s airborne forces that was capable of traversing muddy, marshy, sandy and snowy terrain. Airborne infantry have historically been lightly armed and sometimes struggled against enemy forces equipped with armour. Attempts to level the playing field with glider transported anti-tank guns or even super light tanks could only do so much. Airborne operations during world war two proved light tanks, like the M22 Locust, were out-gunned, under-armoured and largely useless. While light artillery proved effective it lacked mobility and while infantry anti-tank weapons like the Bazooka were extremely useful they were close quarter weapons.
In response the US Army decided to abandon one of the points of the classic ‘iron triangle’ of armoured vehicle design all together, sacrificing protection for firepower and mobility. The unarmoured 16,000 lb (7 tonne) vehicle could be dropped from a transport plane to support paratroops and later heliborne air cav units.
The M56 was developed and manufactured by the Cadillac Motor Car Division of General Motors, a pilot version was completed by 1955. The pilot model, designated the Full-Tracked Self-Proppelled Gun T101 is seen in this photograph from October 1955. When the vehicle finally entered production in 1957 it was largely identical to the T101 except for changes to the location of the radio, the design of the gun’s muzzle device, the hinged flaps on the gun shield were abandoned and the sand skirts were also abandoned. The Scorpion had no secondary armament and no armour. Protection for its 4-man crew amounted to nothing more than a gun shield, which also has a window for the driver.
The 90mm M54 high velocity gun was mounted on a pintle in the centre of the vehicle with the driver on the left and the vehicle commander, loader and gunner sat around the gun. The gun could be traversed 30-degrees left and right and had 10-degrees depression and up to 15-degrees elevation. At the rear of the vehicle was an ammunition store that held 29 90mm rounds. The main ammunition used with the M54 would have been the M318 armour piercing round but it could chamber any of the other 90mm ammunition then in US service. Sadly the ammunition store and breech of the gun aren’t present on this example.
The gun’s impressive recoil, even with a pair of recuperators, hit the Scorpion and the crew manning it hard, the footage shows just how powerful the recoil was. In this contemporary footage we can see that the front wheel almost bounces off the ground. Note also the semi-automatic action that opens the breech and ejects the spent shell casing after the gun is fired.
The vehicle was powered by an air-cooled petrol engine that produced 200hp. Capable of a maximum speed of just over 28mph.
Perhaps its most interesting feature is that it runs on four pneumatic road tyres, rather than metal road wheels, enclosed in a 20in wide track. The track was made up of two bands of rubber and steel cross pieces. There’s a sprocket at the front and an idler at the rear for tensioning the track. This configuration was chosen to reduce weight and it also minimised the ground pressure of the vehicle.
Production of the M56 ended in 1959, after around 325 had been produced. While most of these entered US service, 87 were purchased by Morocco in 1966 while a further 5 were used by the Spanish Marine Corps.
While the M56 was slightly more mobile across country, in reality it wasn’t much better than a standard jeep equipped with a M40 recoilless rifle. Despite the Scorpion’s shortcomings it remained in service into the early 1970s and saw action during the Vietnam War with the 173 Airborne Brigade, which had a company of 16 M56s. In Vietnam the more capable and better protected M551 Sheridan saw wider use and what action the Scorpions did see was largely acting in direct fire support.
Special thanks to Battlefield Vegas for allowing us to film and feature their M56.
A Bridge Too Far (1977) is undoubtedly a classic of the war film genre, massively ambitious it attempts to tell the story of Operation Market Garden. One of the key stories told is that of 2 PARA besieged in Arnhem awaiting relief from XXX Corps.
Perhaps one of the most enduring scenes sees Anthony Hopkins, portraying 2 PARA’s commanding officer Johnny Frost, spot an enemy tank approaching and bark the order: “Bring Up The PIAT!”
The scene itself is actually quite authentic. The PIAT gunner misses, and that isn’t too surprising as despite being a platoon weapon not everyone had a lot of training on them. While the PIAT misses twice – this is because the gunner was firing from an elevated position. This makes judging the range and lead which should be given to an advancing tank all the more difficult. It is something we see in contemporary accounts, including in Arnhem Lift: Diary of a Glider Pilot, by Louis Hagen. Hagen describes firing a PIAT at a self-propelled gun (likely a StuG) from an attic during the fighting in Arnhem: “The direction was perfect, but it fell about twenty yards short.” Similarly, there are accounts from Home Army members fighting in Warsaw during the Uprising which describe exactly the same thing.
While the flash we see in the scene might be excessive the recoil is quite authentic. While writing my book on the PIAT, I did a lot of research into the cultural impact of the PIAT and the numerous films it appeared in since World War Two. I recently wrote an article about the numerous films it has appeared in, you can read that here.
Perhaps the most important and realistic appearance was its first, in the fascinating 1946 film ‘Theirs Is The Glory‘. It’s a unique film that was filmed entirely on location with from veterans of the battle making up most of the cast and help from the British Army’s Army Film and Photographic Unit.
The PIAT appears twice in the film, scene some PARAs are trying to fight through to Arnhem but have been pinned down by what appears to be a French Char B. As a sidenote captured Char B1’s in German service were present in Arnhem).
The PIAT team are seen to move to the flank to get a good shot at Char B. The short scene gives a good indication of how the No.2 would load the PIAT as well as showing the rate of fire possible – a good team could get off five rounds a minute. Theirs Is the Glory also features another brilliant PIAT scene with Corporal Dixon seen knocking out a Panther
I would highly recommend both films as they are both interesting depictions of the battle and both good representations of the PIAT in action.
The transfer of rifles began in the autumn of 1940, with the training pamphlet ‘The Home Guard .300 Rifle P.17 (American Manufacture)’ published in September by the government. Which began “it now appears that all ‘Home Guards’ will ultimately be equipped with this rifle…”
In May 1941, the Home Guard’s .303 rifles began to be withdrawn and reissued to Regular Army units. These rifles were steadily replaced by American M1917s arriving from US stockpiles. This particular rifle was built by Remington in August 1918.
By the spring of 1942, 80,000 M1917s had arrived, the first of 500,000 that were to be transferred. These would go some way to arming the over 1 million Home Guard members who needed weapons.
The Home Guard were stood up in May 1940, initially known as the Local Defence Volunteers, they were a sort of armed citizen militia made up of men ineligible for regular military service. They were formed into local platoons and companies and were initially poorly armed and equipped. But in time became a well-equipped home defence force.
The M1917 has a somewhat complicated origin. The story began with the British Army’s pre-World War One attempts to replace the SMLE. The Pattern 1913 was developed, based on a modified Mauser action and chambered in a new .276 round. Before the P13 could be fully evaluated and adopted – war were declared – and the British government placed contracts with US manufacturers to produce the Pattern 1914, the P13 adapted to chamber the standard .303 round. Due to a lack of parts interchangeability between the P14s which reached Britain it did not see front line service. In 1917 the US entered the war and found themselves in need of rifles quickly. With the production lines for the P14 already in place at Winchester, Remington and Eddystone the decision was made to produce the P14 chambered in .30-06. This was adopted at the Model 1917.
The per unit manufacturing cost of the US M1917 rifle in 1917-18 was only $26.00, they almost certainly cost Britain much more to purchase in 1940. Despite the M1917s being more plentiful in 1918, than the M1903 the US Army opted to retain the M1903 as their primary service rifle. As such the rifles sold to Britain had been in storage, often in cosmoline, for two decades and were in good shape.
As the M1917 was chambered in .30-06, or as the British referred to it .300, the rifles were painted with a red band around the wooden forend furniture to prevent the wrong calibre being used. The same measure was taken with the various Browning M1917 medium machine guns and M1918 Automatic Rifles also chambered in the American round. Some rifles also had a .300 stencilled on the butt.
Home Guard riflemen were to be each issued with fifty rounds of .300 ammunition, but in the early stages of the war ammunition was extremely limited. While this hindered familiarisation with the rifle somewhat, it didn’t hinder rifle training completely as many Home Guard units would have practiced with .22 rifles on miniature ranges and with rifles and ammunition provided at Regular Army Ranges. In this clip from some footage of Warwickshire Home Guard men, we see a corporal happily posing with a .22 Martini rifle.
The M1917 is an excellent rifle and the Home Guard were lucky to have them. While those lucky enough to have received an SMLE may have been disappointed when they were given an American rifle in its place many appreciated the rifle. It was certainly better than the smattering of shotguns, civilian rifles, older service rifles and Canadian Ross rifles some units found themselves armed with during the Home Guard’s early days.
One Home Guard Unit In Denbighshire, Wales was initially issued 100 Canadian Ross rifles between 500 men until, in the spring of 1941, they received M1917s. One rifle for every two men.
Clifford Shore, a member of the Home Guard who later became an officer with RAF Regiment, recalled in his post-war memoir that the M1917s:
“were really splendid weapons; I never came across a bad one. In certain quarters they were not popular, but that can be primarily and summarily dismissed with the one word ‘ignorance’. …The higher velocity .300 cartridge gave slightly improved ballistics than the .303 cartridge in the P14, and I should say that the M17 was probably the most accurate rifle I have ever used.”
The Warwickshire Home Guard In Action
The video features footage of a Warwickshire Home Guard unit. In it we get a rare glimpse at the men at the range with their M1917s. They’re paired up with spotters and instructors and we also get to see the men in the butts running the targets for the shooters.
In another piece of footage of the same Home Guard platoon we see them drilling with their rifles. they’re carrying out muscle exercises. The manual for the ‘.300 Rifle P.17’ lays these out.
The 1st practice trained men how to lift the butt of the rifle into their shoulders and how to level the rifle quickly for aiming. The second was to strength the grip of the hands and the 3rd laid down in the manual trained the soldier to hold the rifle steady while aiming building strength to increase stability.
Examining The M1917
The rifle weighs 9.2lbs (or just under 4.2kg) unloaded, it was 46.25in (117cm) long and had a fixed, internal double stack magazine, which because of the lack of a rim on the .30-06, could hold 6 rounds.
The rifle has a Mauser-style bolt release on the right, pull back on that and slide the bolt out. The rifle has an aperture rear sight, zero’d for 200 yards, with a peep also mounted on a ladder giving graduations out to 1,600 yards.
The bolt of course has the dog-leg handle which was carried over from the P14, which in turn emulated the SMLE’s bolt handle position – falling nicely under the hand.
Unlike the earlier P14, the 1917 dispensed with the volley sights seen on the British rifles. The action is cock on close and the bolt itself is based on the Mauser 1898’s.
This rifle was manufactured by Remington in August 1918. By the end of production Remington along had produced 545,541 rifles. At peak output almost 10,000 rifles were being produced per day, with the final number built standing at 1,727,449.
I’m very excited to say that my second book has been published! It looks at the much maligned and much misunderstood PIAT – Projector, Infantry, Anti-Tank.
The book is available from retailers from the 20th August in the UK/Europe and the 22nd September in the US – but you can order a copy from me now regardless of location. I filmed a short video to show you the book and talk a bit about the process of writing it, check that out above.
The PIAT was the British infantry’s primary anti-tank weapon of the second half of the Second World War. Unlike the better known US Bazooka the PIAT wasn’t a rocket launcher – it was a spigot mortar. Throwing a 2.5lb bomb, containing a shaped charge capable of penetrating up to 4 inches of armour. Thrown from the spigot by a propellant charge in the base of the bomb, it used a powerful spring to soak up the weapon’s heavy recoil and power its action.
With a limited range the PIAT’s users had to be incredibly brave. This becomes immediately obvious when we see just how many Victoria Crosses, Military Medals and Distinguished Conduct Medals were awarded to men who used the PIAT in action.
The book includes numerous accounts of how the PIAT was used and how explores just how effective it was. I have spent the past 18 months researching and writing the book and it is great to finally see a copy in person and know it’s now available.
The book includes brand new information dug up from in-depth archival research, never before seen photographs of the PIAT in development and in-service history and it also includes some gorgeous illustrations by Adam Hook and an informative cutaway graphic by Alan Gilliland.
It’s immensely exciting to know the book is out in the world for all too enjoy. If you’d like a copy of my new book looking at the PIAT’s design, development and operational history you can order one directly from me here!
Thanks for your support and if you pick up a copy of the book I really hope you enjoy it!
In this very rare footage we see a Free French Air Force officer, possibly training as a member of the SOE, place a limpet mine on a substantial piece of metal plate.
The mine seen in the footage is clearly much smaller than the Limpets used against ships. The Limpet mine was developed by Military Intelligence (Research) in late 1939-40. Stuart Macrae and Cecil Vandepeer Clarke developed a mine with enough magnetic strength to attach an explosive charge to the hull of a ship. The initial design seen here was quite large but the design was refined as the war went on with various types and marks. Here’s a Type II limpet, a MkIII and here is a Type 6 MkII.
The idea was that divers or saboteurs in small boats could quietly attach the mines to enemy shipping while at anchor. However, the usefulness of magnetic charges was clear and it appears that smaller versions, like that we see in the footage here, were developed for use against armoured vehicles and other substantial armoured targets.
It’s unclear from the film what the explosive charge was, how big it was or how it was laid out inside the mine but from the damaged plate displayed at the end of the footage it may have been a ring of plastic explosive held in place by the four magnets. This would blow the characteristic round hold in the plates.
Interestingly, the limpet mine seen in the film is very similar to a Japanese design, the Type 99 anti-tank mine, however, it has a different fuse design and the four magnets are blocky rather than rounded. Whether the Japanese magnetic mine influenced this design developed by SOE is unknown.
I’ve been unable to find out these mine’s designation, it may not have been given one but it does appear to be fairly well developed. In this photograph we can see that a metal plate carrier has been developed to allow a soldier to carry 4 mines on his back. Perhaps these mines were developed for a specific mission. The magnetic Clam charge, which we have covered in an earlier video, would have done a similar job for smaller task
Destroying railway infrastructure was a key mission for the Resistance groups and SOE agents active in occupied Europe. Numerous methods of damaging or destroying railways were developed, including Exploding Coal, which we have covered earlier in this series. In this 16mm colour footage, believed to have been filmed in 1940, we get an early look at the methods the SOE were developing to destroy track. The ultimate aim was to derail the locomotive and wreck the train with minimal effort and explosive.
In the footage we see two charges have been placed on the piece of track, with detcord attached to both. A soldier, with what appears to be a lever-action Winchester 94, is then seen taking aim. It seems he’s aiming at a striker board attached to ignite the detcord. He fires, we see a puff of smoke and a second later the charges detonate.
The footage then cuts to several men collecting the debris of the shattered piece of track. The track appears to have two large chunks blown out and the top edge, between the two charges, completely blown off.
Later in the war more testing was done and more refined techniques were developed. In their book SOE: The Scientific Secrets Boyce & Everett note that trials of devices and techniques for destroying railway lines carried out at Longmoor where the British Army had extensive sections of track and samples of rails used in different European countries. Trials to find the right quantity and positioning of explosive charges were carried out in late December 1943, these tests would inform later operations.
The SOE’s Descriptive Catalogue of Special Devices and Supplies includes a pair of illustrations demonstrating two methods of laying and detonating these charges. A so-called ‘French’ method with a pair of what the catalogue terms ‘Igniters, Fuze, Fog Signal, MkIA’ ahead of the charges in the direction the train was expected from. The train would crush these Fog Signals firing them and igniting a length of detcord linked to a pair of 3/4lb explosive charges fixed to the track as we see in this film.
The alternative ‘Polish’ method had the same sized and located explosive charges but placed a Fog Signal either side of the charges to ensure that no matter which direction the train came from the charges would be detonated. This method was used on single track stretches of railway. Both of these methods were rated to ‘remove about one metre of rail.’
In this photo we see a member of the French Resistance setting an explosive charge on a railway line. While likely a posed photo we do see the pair of Fog Signals which will stet the charge off. These photographs show a pair of trains reportedly derailed by explosive charges.
Boyce & Everett in their book SOE: The Scientific Secrets suggest that as many as 48,000 ‘Railway charges’, presumable a kit, were produced by the SOE. From the footage we can certainly see this method of destroying rails was effective.
Today, were taking a look at a Winchester prototype developed in the mid-1860s, a period when Winchester was seeking to build on the success of the 1860 Henry Rifle and place the company on a firm financial footing. Oliver Winchester had taken control of the New Haven Arms company before the Civil War and while for a time it had been known as the Henry Repeating Arms Company he eventually sought to put his stamp on the company, renaming it Winchester Arms Company in 1866. At the same time he decided to focus the company’s energies on winning military contracts around the world.
This developmental prototype is in the ‘musket’ configuration: with a longer barrel, a bayonet lug and a wooden forend. The prototype represents one of the many developmental steps towards what would become the Model 1866. It has a number of interesting features – a steel, rather than brass, receiver and a hinged loading port developed by Nelson King, Winchester’s superintendent between 1866 and 1875.
The rifle itself was built by Luke Wheelock, Winchester’s model room mechanic and a designer in his own right who would go onto develop his own rifle designs for Winchester.
The rifle is 54.5 inches long, with a 33.75 inch barrel. Believed to have been built in 1866, it is chambered for a .45 calibre rimfire round. King patented his loading port in May 1866. He described how the port worked:
“Through one of the plates S (preferring that one upon the right-hand side) I form an opening, 0, as denoted by broken lines, Fig. 1, and also seen in section, Fig. 7. This opening is formed so as to communicate through the frame directly to the chamber E in the carrier block, as seen in Fig. 3. Through this opening, and while the carrier-block is down and all parts of the arm in a state of rest, insert the cartridges, point first, through the said opening in the plate S into the chamber E the second cartridge pressing the first into the magazine, and so on with each successive cartridge until the magazine is filled, or until the requisite number has been inserted therein, the follower G being pressed up before the entering cartridges. In the rear of the chamber E2 the frame forms a shoulder to prevent the cartridges from being forced out through the opening in the plate S3 is a cover for closing the opening in the plate S3 and is hinged thereto, as seen in Figs. 1 and 7, the hinge being provided with a spring,a1, the tendency of which is to open the cover C. A spring-catch, d, (see Fig. 1,) secures the cover when closed, so that by pressing upon the said catch the cover will fly open. After the requisite number of cartridges have been placed within the magazine, close the cover, as seen in Figs. 1 and 2.”
To paraphrase: ammunition can be loaded through the opening in one of the receiver side plate when the carrier block is down, insert the cartridges through the opening, pressing the first into the magazine and so on until the magazine is filled… a cover for closing the opening is hinged to the receiver side plate. A spring catch secures the cover when closed.
According to Herbert Houze, King developed the covered loading port design in early January 1866, with a design drawing dating to the 14th January, confirming this.
King altered the design of the rifle’s cartridge carrier so that a cartridge could pass through its lower section straight into the magazine when the action was closed. In theory the aperture could be placed on either side of the receiver, in practice is was placed on the right. Prior to this Winchester had experimented with systems where the tube could slide forward (G.W. Briggs US #58937), a port in the base of the receiver (J.D. Smith US #52934) or a sliding forearm covering a loading port at the rear of the magazine tube (O.F. Winchester UK #3284 [19/12/1865]).
King’s system had the benefit of allowing the rifle to be quickly loaded or topped off without rendering the rifle unusable while loading. Positioning the port in the receiver allowed the magazine tube to be enclosed by a wooden forend.
A cartridge guide was fitted inside the receiver which guided rounds through the cartridge carrier and into the tube magazine. The rounds were prevented from popping out of the magazine, when the carrier was aligned and the cover open, by a shallow shoulder which projected in line with the carrier’s channel to hold cartridges in the tube by their rim.
The hinged cover is held shut by a spring catch mounted on the rear of the cover. When the knurled section on its front is pressed rearwards the cover pops open. The spring catch is actuated when it tensions against the cover’s hinge as it is closed. On the back of the cover there is also a cartridge stop for when the cover is closed.
Another small but interesting feature of the prototype is the catch at the rear of the lever loop, this differs from the manually turned catch seen on the Henry and production 1866. This design appears to be a much better safety feature, simply requiring the user’s hand to depress the catch to unlock it from the stock. It also appears to be a much simpler mechanism than that seen in later models like the Model 1895. The trigger also had an extension protruding from its rear which appears to prevent the trigger from being pulled when the lever isn’t full closed. Neither of these features appear in King’s May 1866 patent.
It appears that the idea of the port with a hinged cover was superseded by what we now recognise as the classic Winchester loading gate in the summer of 1866. King’s new system replaced the hinged cover with a piece of stamped spring steel attached to the inside of the receiver side plate by a screw. The spring steel gate could be pushed in, with the nose of a cartridge, to allow rapid loading. The front face of the gate formed a cartridge guide removing the need for the separate machined guide used in King’s earlier iteration of the system.
King’s revised loading port system required just five, rather than twelve, components: King’s altered cartridge carrier, receiver side plate, spring metal loading gate plate and retaining screws. This simple but elegant design continued to be used for decades on various models of rifle. The company were so pleased with the refinement of the rifle that, according to R.L. Wilson, King was awarded a payment of a $5,000 reward by the company’s board of directors.
Winchester introduced the rifle in 1866, with the first deliveries being made early in 1867, the new rifle was offered in various barrel lengths and patterns including carbine, rifle and ‘musket’. Winchester found some success selling 1866 rifles to the militaries of France and the Ottoman Empire, while many other countries purchased rifles for testing including Britain and Switzerland (whom came close to adopting the Winchester.) The rifles also found success on the civilian market with around 4,500 sold in the first five months.
The Scientific American described the new rifles as “elegant in appearance, compact, strong, and of excellent workmanship. On examination we find its working parts very simple, and not apparently liable to derangement.”
King incrementally developed his loading system before radically simplifying it and this prototype rifle represents an important developmental step in the design of what would become the Model 1866 – one of Winchester’s most important rifles.
Special thanks to the Cody Firearms Museum for allowing us to take a look at this fascinating prototype rifle.
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Winchester Repeating Arms Company, H. Houze (1994)
Winchester: An American Legend, R. L. Wilson (1991)
The footage, believed to have been filmed in 1940, is part of the Imperial War Museum’s collection, it shows plastic explosive being demonstrated in a number of different applications. It was filmed by Cecil Vandepeer Clarke, a British engineer and sabotage expert who was a member of the Special Operations Executive and worked at a number of weapon research and development centres during the war.
The clip features a number of men preparing and shaping plastic explosive charges, adding fuses and detonators. The explosive is then seen being applied to a steel plate in a ring shape, before being detonated. The resulting explosion punches a round hole through the plate. The film also includes demonstrations of what plastic explosive pressed against a tree trunk can do. Once detonated the roughly 1 foot thick trunk is splintered in two. Metal girders are also shown being prepared with a substantial block of explosive being pressed into its seams.
The SOE’s 1944 Descriptive Catalogue of Special Devices and Supplies lists the ‘Standard Charges’ of 1.5lbs or 3lbs of plastic explosive with an integrated central primer available in rectangular blocks inside a rubberised fabric. Of course SOE agents were taught to use as much or as little explosive as was needed for the task and they were taught to be able to improvise in any given situation.
Given the date of the footage the explosive being used is likely and early form of plastic explosive produced at Woolwich arsenal, possibly PETN or Cyclonite better known as RDX, which would have been mixed with a plasticiser to make the explosive malleable.
Today, we’re lucky enough to have some colour footage showing the of testing of a magnetic bomb which could be attached to the petrol tank of vehicles. The footage comes courtesy of the Imperial War Museum.
From the film we can see that the bomb consisted of a small block of plastic explosive, a pair of strip magnets (or possible a horseshoe-shaped magnet) and a Switch No.10 time pencil delay detonator. The explosive block itself looks to be slightly smaller than the SOE’s standard 1.5lb charge.
In the film we see the bomb placed on the boot (or trunk) of a saloon car before various civilians and a corporal experiment with various ways of covertly attaching the bomb to the underside of the car. At one point the corporal allows himself to be dragged along behind the vehicle before making his escape.
Luckily the 16mm footage, filmed by Major Cecil Clarke, also shows us the effect of the explosive charge mounted on a petrol tank full of fuel. According to the details listed for the film by the Imperial War Museum the footage was filmed in 1940, at SOE Station XVII, located at Brickendonbury House in Hertfordshire.
This configuration of the bomb doesn’t appear in the Special Operations Executive’s Descriptive Catalogue of Special Devices and Supplies published in 1944. However, Colonel Leslie Wood, Station XII’s commanding officer, described the demonstration put on during a visit by Brigadier Robert Laycock of the Commandos and William Donovan, the head of the American OSS in June 1942. One of the scheduled demonstrations was the “Effect of small ‘magnet’ charge of explosive on petrol tank of car.”
It appears that this ad hoc magnet charge evolved into ‘the Clam’, which was a smaller, version of the magnetic Limpet mine. The Clam evolved through a number of marks with the MkI having a stamped sheet metal casing and the later MkIII using a bakelite, plastic casing. Both were made up of a plastic explosive charge inside a rectangular, rounded case with a pair of magnets at either end. They were detonated by either a Time Pencil or an L Delay fuse attached to a No.27 detonator. The MkIII had 8oz (226g) of high explosive filler, such as TNT/Tetryl 55/45.
While unlike the larger Limpet they weren’t developed for under water use but the Clam could be mount onto any vaguely flat magnetic surface including engine blocks, fuel tanks, crank cases, cylinder blocks, rail tracks and steel plate.
At just 5.75” x 2.75” x 1.5” they were easily concealable, could be carried in a pocket and were non-descript enough not to draw attention. An estimated 68,000 Clams were made under supervision at Aston House according to Des Turner’s book on Station XII.