This is what I understand -- Many years ago, the (then) ATTD (now BATFE) listed the six parts they considered necessary to either convert an M1 carbine to selective fire or that in themselves constitute a machinegun within the meaning of the term in the National Firearms Act.
They are: 1) M2 hammer, 2) M2 sear, 3) disconnector, 4) disconnector lever, 5) selector, and 6) M2 slide (cut out on right side to operate the disconnector lever).
Note that the M2 trigger housing, M2 stock, round bolt (slows down rate of fire), and small parts (springs) are NOT included.
The three parts that are exclusive to selective fire and have no place in a semi-auto are the disconnector, disconnector lever, and selector. The M2 hammer will work in an M1 with a shim. Many M1 carbines sold through DCM and CMP had or have M2 slides and M2 sears, as well as M2 trigger housings.
