Mfg in 1944. The M3A1 was used during World War II and Korea. Basically the same as the M3 with the following differences:The M3A1 housing assembly has been changed in design to eliminate the retracting handle, spring, lever assembly, and oiler clip. It contains the ejector, which is secured to the housing by two rivets.*
When 9mm ammo was available.
The M3A1 was also designed so it could fire 9mm Parabellum cartridges by changing the barrel and bolt and adding an adapter to the magazine.