If it's simply a slide and not in of itself a F/A part (or a part that makes the gun F/A), then I'd almost have to think that it would be one of those situations where it (pre-86) could have been used on a gun in conjunction with an unregistered conversion device (e.g. trigger assy).

Though, perhaps by applying the logic used on the H&K type weapons, if doing the F/A conversion would in no way leave extra parts around that would facilitate the creation of a new MG, perhaps you could install it in an M1 without issue and have yourself an M2.

I hate to suggest it, but perhaps (in this specific case) a determination letter may in fact be appropriate if ATF hasn't already issued other clarification with regards to M2's.