This is the M56 Scorpion SPAT or Self Propelled Anti Tank Gun. It was produced for the US Army from 1953 to 1959 by the Cadillac Motor Car Division of General Motors and intended to arm Paratroopers and other light infantry formations with a light and highly mobile system to engage enemy armor.
It mounted a very capable 90mm gun, very similar to the one used on the M48 Patton tanks used in that era. 29 rounds for the gun were carried on board. The gun had a maximum range of 1,500 yards. The vehicle weighed in at 14,000 pounds and could be air dropped from fixed wing aircraft or set in place via a helicopter sling load. The engine was a gasoline powered Continental 200 horsepower, capable of a range of 140 miles and speeds up to 28 miles per hour. The gun was operated with a crew of four: commander, gunner, loader and driver.
The M56 Scorpion served in the 82nd and 101st Airborne Divisions, seeing service in the Vietnam War as fire support platform. It was replaced by the M551 Sheridan light tank.