Apparently there was a Supreme Court decision, US vs Haynes in 1968.
Haynes was not legally supposed to own a firearm.
But he did. (he was a felon)
Where he lived required registration.
If he registered, it would mess up his 5th amendment rights against self incrimination.
What to do?
Only the law-abiding can be required to register their guns.
We hold that a proper claim of the constitutional privilege against self-incrimination provides a full defense to prosecutions either for failure to register a firearm under sec.5841 or for possession of an unregistered firearm under sec.5851.