Hmmm ... interesting ... I've never heard that designation before. M-1 carbine was the original, semi-automatic version and the M-2 was the selective fire (semi-auto/full auto) version.
The M-1 carbine was simply meant to be a light weapon for support troops that wouldn't weigh them down but would still give them greater firepower than just a sidearm. The cartridge is underpowered, but it was better than just carrying a .45 1911 pistol.
The essential American soul is hard, isolate, stoic, and a killer. It has never yet melted. ~ D.H. Lawrence