In CA, you don't need to do any of this 'keep at least one part of the magazine pre-ban' voodoo.
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.) The restriction applies to both centerfire and rimfire weapons.
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*(see footnote), 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.
The age of the magazine and the age of the gun mean nothing. The critical item is possession of the magazine(s) in California before January 1, 2000.
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. 'Importing' ALSO applies to moving here or visiting here.
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.
Yes, you can take one of those kits and create a 10-round magazine from the parts and some method of blocking the capacity; one example here.
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."