When I first read your previous post, I thought maybe it had something to do with transferable M2 kits.
There is some debate in the Carbine community about the status of registered trigger packs. Some people claim that the registered M2 trigger housings put out by Fleming, Norrell and others were improperly registered (because they weren't one of the seven full auto parts exclusive to the M2 Carbine) and thus contraband. Others say these kits are in a legal gray area. Still others say that there is nothing to be concerned about. The debate doesn't appear to be going away. I suppose that one day, somebody will ask the ATF flat out what the story is with these registered packs.