19462  The T30 HMC was originally conceived in 1941 as an interim design to fulfil the Armored Force 's requirement for an assault gun to equip tank and armored reconnaissance units . The Ordnance Department design was based on the M3 Half @-@ track in order that it could be brought into service quickly . A prototype vehicle was authorized in October 1941 armed with an M1A1 75 mm Pack Howitzer and a mount that was designed to fit on a simple box structure in the back of an M3 Half @-@ track . 