Ahhhh, another interpetation of the law that is self serving.
If you are not covered by one of the exemptions (see some of them below) ...
If you did not possess a large-capacity magazine in California before January 1, 2000, you may not have it now
If the gun (and its matching magazines) did not exist before January 1, 2000, you shouldn't have large-capacity magazines for it now (e.g. Springfield XD, first marketed in 2002 [as distinct from the 1999 HS2000]. You shouldn't be prosecuted for possession/use, but the timing can't be explained
A contrary example: a legally possessed in California pre-1994 Para-Ordnance magazine may fit a post-2000 Para pistol - the gun didn't exist before 2000, but a magazine that fits the gun did -- This is a gray area, you may have this one - no logical contradiction.)
No one in or out of California may transfer a large-capacity magazine to a non-LEO California resident
The relevant Penal Code is 12020(a)(2)
manufacture or cause to be manufactured
import into the state
keep for sale
offer or expose for sale
Note that it is not prohibited to
own a large-capacity magazine
possess a large-capacity magazine
use a large-capacity magazine, in whatever gun*
repair a legally possessed large-capacity magazine
(and, necessarily, to acquire the parts to make the repair --
remember it only makes sense to buy parts kits for existing
legally-possessed large-capacity magazines)
(* Old magazine, new-ish magazine, old gun, new gun - all irrelevant; remember also the 10-round limit on fixed magazine OLL, a different law.)
Large-capacity magazines manufactured during the now-defunct Large Capacity Ammunition Feeding Device ban of 1994-2004 were marked 'Law Enforcement Only'. There are reasons one might legally have possessed them in California before 2000, but it is also legal to have the magazine bodies as repair parts for your other magazines.
There are exceptions in 12020(b), (b)(19)-(32) notably
(23) The importation of a large-capacity magazine by a person who
lawfully possessed the large-capacity magazine in the state prior to
January 1, 2000, lawfully took it out of the state, and is returning
to the state with the large-capacity magazine previously lawfully
possessed in the state.
(b)(19) exempts LE agencies
(b)(20) exempts LEOs
(b)(21) exempts CA-licensed FFLs
(b)(22) exempts loans so long as the lender stays nearby
(b)(27), (b)(28) and (b)(29) exempt "entities that operate armored vehicle businesses" who buy large-capacity magazines, lend them to their employees or receive them back
The crux of this is INTENT and common sense, replacing a magazine body with a PMag body will probably not fly as a repair...