The footage featured in the video above was filmed at the 1923 US Army Ordnance Exhibition of Ordnance, where John Walter Christie demonstrated his latest amphibious vehicle.
Christie’s amphibious vehicle is one of the earliest. It follows the amphibious variant of the British Mark IX tank, essentially an amphibious armoured personnel carrier (armed only two machine guns) and preceded by the Vickers-Carden-Loyd Light Amphibious Tank in the early 1930s.
More of a gun carrier than a tank, as the vehicle was open topped, it was, however, equipped with a 75mm field gun. During the footage it not only seamlessly operates on both land and water, it also fires four rounds as it crosses a body of water at the Aberdeen Proving Ground.
Very little is known about the vehicle but it is believed to be the second of three amphibious vehicles developed by Christie during the 1920s. None of the vehicles were purchased by the US military and no major international orders were made either.
The vehicle itself is manned by two people during the demonstration, presumably one steering while the other mans the gun. It appears to have narrow tracks over its four sets of wheels and a pair of propellers at the rear.
Bibliography:
Demonstration of Ordnance Materiel at Aberdeen Proving Grounds, 1920-1926, US National Archives, (source)
The Armourers Bench 