The Red Army experienced a need for light SPGs armed with 76 mm guns as early as Fall of 1941, but that need was not met until December of 1942, when production of the SU-76 SPG began. Sadly, production was dragging slowly, and the vehicle ultimately ended up a bust. The need for a light SPG didn't go anywhere. In order to meet the army's needs, factory #37 converted PzIIIs and StuGs to use an F-34 76 mm gun in a fixed casemate.
"According to GOKO decree #2758 from January 18th, 1943, factory #37 and TsAKB NKV designed and built an experimental prototype of a SU-76I SPG on the chassis of a captured StuG SPG and PzIII tank, using a stock 76 mm F-34 tank gun.
In March of 1943, the vehicle was tested at the Sofrino proving grounds over 280 kilometers of travel and 434 rounds.
The experimental prototype passed trials, and was recommended for service in the Red Army. The SU-76I SPG was accepted into the Red Army by GOKO decree #2758ss from January 18th, 1943."