I'm the Director of Compliance for a FFL/Gun Range in California, maybe I can shed some light on it.
California bans the manufacture, transfer, blah, blah, blah of "high-capacity" magazines (anything over 10 rounds) but, however, does not ban the possession of said magazines. So long as you are the original owner of "high-capacity" magazines and you've owned them prior to January 1st, 2000, you may still own and use your magazines. Should any of your magazines become un-serviceable or damaged, you may legally replace or repair your original magazines using a legally-available "re-build kit." A re-build kit is essentially a fully-functional 30 round magazine, just disassembled. So long as it's not built and disassembled, you can openly and legally sell/transfer a re-build kit in the good faith that the buyer is the owner of legally-grandfathered magazines.
You may also use the re-build kits to make "ten-thirty" mags, which are mags that originally started as 30 rounders but are permanently modified to only take 10 rounds. The permanent part is important to note. Simply putting a block or spacer in a 30 rounder, then putting it back together is NOT CA-legal because it is not permanent. The penal code covering high-capacity magazines does not state exactly how it's supposed to be permanently fixed, but it states that it must be permanent. Some of the methods I've seen are epoxied floor plates on polymer mags, rivets, welds, and other ways of sadly making 30 rounders permanent 10s. If a cop wants to check, he can try to fit more than ten. If it even gets to that, most cops will stop there if it only holds 10 rounds. Some cops in the know, who want to give you a hard time, may try to take the mag apart, and God help you if you didn't permanently secure that mag -especially if you're using it in a bullet-button AR, because then you've just committed a felony.
Going back to the magazines, there is no provision in the law that requires you to keep receipts of purchase for your original mags -it's a toothless law. There are some restrictions however; you may not use your legally-grandfathered magazines in fixed-magazine weapons, example: bullet-button equipped AR15. You can legally have an off-list, bullet-buttoned AR15 and you can have your legally-grandfathered 30 rounders, but the second you insert magazine into rifle, you've just "manufactured" an "unregistered assault rifle."
Vote accordingly to ensure this madness doesn't happen to your state. Don't tune us out -learn from California's sad times. The Feds use this state as a proving ground.