Re: m-2 carbine
Yes this is correct. I was a class 3 dealer for many years and I still own an M2 marked carbine. The receivers on M1 and M2 are identical except for the number stamped on the bridge of the receiver. The trigger housing and several internal parts are different, and there is a selector switch, selector spring, and disconnector lever added to make the carbine fire in full automatic mode.
Removal of the M2 parts does indeed convert an M2 back to legal semiautomatic status ONLY IF the receiver is marked M1. If the receiver is marked M2, it was originally manufactured as a machine gun and it will always be considered a machine gun by BATF. It's common to find carbines with M2 type trigger housings, sears and hammers, even disconnectors included in the trigger housing, but the selector switch, selector spring, and disconnector lever are ALWAYS missing from any M1 carbine I have ever seen.
So, if the carbine you are considering purchasing is marked M2 on the receiver, forget about it. It's not worth 10 years in Club Fed. No way to ever make it legal either.