I am not sure how much effect the sales slip (or DCM records) would have, but BATFE has ruled that a carbine marked M2 or M11* is a machinegun, whether it has the M2 parts or not. This situation is tricky, and the only way to get a definitive ruling is a letter from BATFE or a court trial, either of which might be undesirable.
*The M11 or M1I marking was used by depots which converted M1 carbines in the Korean war period. They just stamped an extra "1" after the original, creating the Roman number "II". They also sometimes stamped a "2" over the "1". Per BATFE, either one makes the carbine a machinegun, and that designation cannot be undone.