an M16 lower with all the holes drilled and material removed for the auto sear and disconnector is definitely a registerable item.
It's definitely not registerable. Not unless BATFE initiates some kind of amnesty program. It is just a one way ticket to Prison. On the other hand, removing the hardened steel block that Colt put in its lowers is not illegal. I've heard it is difficult, though, and could hurt the value of a Colt firearm.