The wiki has a long article: Large-capacity magazine restrictions and there is a long thread here at Calguns
but for those people who find themselves unable to click on a link:
California restricts certain things about magazines greater than 10 rounds; it has created a silly legal name for them - "large-capacity magazines" - and says we 'regular people' cannot import, sell or manufacture them. ("Dealer" FFLs may get licenses from DOJ for these activities; curio and relic licensees cannot.)
It is perfectly legal - no restrictions at all in the Penal Code - to own and possess and use those "large-capacity magazines" in any gun*, at any time, under any circumstances where it is legal to use a gun. Ownership, possession and use of "large-capacity magazines" are not crimes in PC; there is no section under which you could be charged.
Those who own "large-capacity magazines" are not required to keep records or receipts, and not required to explain how they got them.
Yes, you can go out of state and buy magazines of any capacity - but you may not bring "large-capacity magazines" back to CA as complete magazines - that would be importing and that can be charged as a felony.
Yes, you can buy part kits to repair your existing magazines - but you cannot assemble them into complete "large-capacity magazines" inside CA - that would be manufacturing and that can be charged as a felony.
You can have "large-capacity magazines"; no one may give them or sell them to you in California, and you may not give or sell them to anyone else in California. This includes sales from LEO and intrafamilial transfers - that would be included in selling and that can be charged as a felony.
There is no such thing as a "pre-ban" or "post-ban" "large-capacity magazine", and it doesn't matter. Markings on magazines have no legal meaning.
If you think your question has not been answered, follow the links at the top of this post and read the articles, then re-read items (2) and (3) above several times.
* Note: it is not a part of the "large-capacity magazine" law, but part of the 'assault weapon' law that says a semiautomatic, centerfire rifle with a fixed magazine that holds more than 10 rounds is an 'assault weapon'.
That means that
if your otherwise legal semiautomatic, centerfire rifle has a magazine lock, such as a 'bullet button',
and you use a "large-capacity magazine" in that weapon, (which, because of the magazine lock is effectively a fixed magazine rifle)
then you will have manufactured an 'assault weapon'.
That's a felony. Don't do that.
Magazines: all the answers you need - Calguns.net