A simplified Thompson with a folding stock and a vertical fore-grip where you insert the magazine.
At the end of WW2, both General motors and Auto-ordnance Company answered to the US tender for a new submachine gun: the Grease gun for GM and the M1947 Thompson for Auto-ordnance Company.
The Grease gun (OTL m3 sub) was finnaly rejected when the M2 Thompson was found to have an higher accuracy.
While at first barreled for .45, the M2 was finnaly re-chambered for 9mm para when it was found to give it superior range and accuracy. The M1947 Thompson would make its name in Korea but mostly in Vietnam, with the teething issues of the M-16, its sturdiness and reliability would give it an almost mythical status.
(as indicated, all the bases used were made by Darth Panda)