Well it used to be that if we “rebuilt” a 10rd mag into a 30 round mag we could keep it. Or if you bought a 30 round mag in pieces and put it back together yourself you could keep it. That loophole has been closed, sadly.
Nice try… The way I understand things that was never so. If you owned a “high/large” cap mag pre ban you can rebuilt that mag using new parts but ‘causing to exist’ a new “high/large” capacity mag was never (post ban) legal. Those “rebuild-kits” that are now illegal was for building compliant mags CA 10 rnds. I think the ban came in effect in 1999.