While the M3 was a simple .45ACP burp gun popular with the late-WWII regular GIs of the day, it could be improved and made even more economically.
Built as an uber cheap alternative to the really nice but kind of expensive Thompson M1A1, the compact M3 was termed the “Grease Gun” by the troops due to its resemblance to that common shop tool. The gun was really neat in a lot of aspects — such as the fact the wire stock also serves as a barrel removal tool, mag loader and cleaning jag. However, the basic M3 could be tweaked and further simplified — hence the development of the M3A1, a story well told in the above short from American Rifleman TV.
The $21 M3A1 in end not only replaced the Thompson in U.S. service but also remained in use with Army armored crews through the Cold War.