He's right, it's the M56 Scorpion, also called the "SPAT" (self propelled anti tank). It was an airborne anti-tank gun. I believe it even had air in the road wheels to cushion the vehicle when it landed. It was little more than an anti-tank gun sitting on a motorized chassis.
The Third Herd (173rd Airborne) used them in Vietnam.
Revell did a 1/40th scale kit of this years ago that has been reissued a few times. Might even be in their recent classiscs line. Before that, I remember it being reissued in the early 80s with their "kit on a pile of dirt" boxart.
Edit: here's the box art of the Revell kit I built about 30 years ago (note, the kit above is NOT mine).
