Please don't confuse us poor old souls by referring the the carbine as the "M1". The term "M1" alone ALWAYS referred to the M1 Rifle, which some folks call the Garand, a term never used by the troops.
Otherwise, the term "M1" is followed by something else, like "M1 Carbine", "M1 tank", etc. The carbine was always called simply the "carbine" (rhymes with "bean", not "sign"); the terms M1 Carbine or M2 Carbine were not used unless there was some reason to distinguish between them.
The folding stock version was the M1A1 Carbine; they were intended for issue to company grade officers in parachute units, although some others "requisitioned" them.
M2 Carbines, whether original or converted, are machineguns and controlled under the National Firearms Act. A receiver marked "M2" or "M11" is considered a machinegun even though it may not have the parts that allow full auto fire.
Jim