It seems that you can possess and use >10 round mags if you owned them in California prior to 01/01/2000. If you did not, you can neither possess or use them. You cannot, for example, buy, import, or manufacture in California any >10 round mags after 01/01/2000. Doing any of that is a felony.
You could expect that if you are found in possession of >10 round mags you could be suspected and charged for illegal possession. It's not clear how the authorities could prosecute you for that if you contended that you legally possess the mags prior to 01/01/2000. It would be useful to have dated receipts of purchase, but if you don't have them, the burden of proof that you didn't would be on the State. Of course, if you didn't reside in Ca prior to 01/01/2000, the State would be justified in assuming that your possession of the mags is illegal. Also, if the specific mags were clearly manufactured after 01/01/2000, you will be in trouble.
Folks have opined about mag "rebuild kits." If you legally possess a >10 round pre-ban mag, you may "rebuild" it. The problem is purchasing new components to convert a 10-round mag to >10 round capacity with these new components. That would clearly be illegal.
That's just my take on this. IANAL, but the lawyers among us may wish to comment...