I think your assessment is correct. AFAIK, there’s no legal definition of an 80% lower, and the only cases I know of that have tested the idea had to do with co

I think your assessment is correct. AFAIK, there’s no legal definition of an 80% lower, and the only cases I know of that have tested the idea had to do with completely milled lowers which were then filled with plastic to make it easier to mill. The feds didn’t like this, so I don’t think those are sold commercially any more. I believe OP is right, though I’m not a resident of CA: you can have a non-functional piece of metal or a registered functional “firearm” (which the feds say is the lower). A functional, unregistered lower would be against CA law. In other states, like neighboring AZ, there’s no issue purchasing an unregistered, non-functional lower, finishing it into a functional piece, and then using it for private use (including non-commercial sale).