The "machine gun part" is a "machine gun" by definition. If you don't have paperwork for the "machine gun part" which is a "machine gun" regardless of whether you have the rest of the parts to make a functional rifle, you're in big trouble.
That's not necessarily the case, but it's the safe way to approach this mess.
Specifically the pre-81 AR sears. ATF ruled that drop in sears made BEFORE 1981 were grandfathered and NOT considered machine guns in and of themselves.
So, you CAN legally own a pre-81 sear, but NOT if you also own a host rifle. Knowing the ATF they would consider it a crime even if you had a pre-81 sear and a rifle with an SP1 bolt carrier. Those bolt carriers will not trip the drop in sear, so it won't do anything, but I wouldn't trust ATF to know that or care.
Owning the sear itself is not a crime, nor is it possession of a machine gun. Crazy since it would be useless without a host rifle, but technically legal. Of course the burden of proof is on you to prove that it was made before 1981, that's gonna be REALLY hard to do.
A post-81 sear is a machine gun in and of itself, and illegal if no tax stamp.
So, a 45 cent piece of metal made in December of '80 is legal, the same piece made in January of '82 would be a felony.
Clear as mud, which is how the ATF seems to like it.