From the release:
It was developed on the chassis of the then-new Light Tank M5 (Stuart VI). The test vehicle had the standard M5 turret removed, and replaced with an open-topped turret.
Armament consisted of a new open-topped turret armed with a 75 mm M2 howitzer, later a 75 mm M3 howitzer, which were reworks of the M1A1 pack howitzer. It carried 46 rounds of 75 mm ammunition; types of ammunition carried were Smoke M89 and H.E. (high explosive) M48. The only other armament was Browning .50-caliber machine gun for local area, and anti-aircraft defense; 400 rounds of .50-caliber were stowed on board.