When the 1968 amnesty was in effect, an acquaintance was sure it was a government trap to find and confiscate auto weapons, so he did not register his. Of course, (if he is still with us) he has a gun that can never be sold or even shown to anyone and is subject to destruction if seized.
Too bad; it was an M1921 in the hard case, with a 100 round drum, and five 20 round sticks, tools, etc. A beautiful gun. I hope it went or can go to a museum instead of the steel mill.