For starters, there's no such thing as a pre-ban large-cap mag (with regard to CA law) as there was never a ban.
However, if you possessed a large-capacity magazine in CA prior to 1/1/00 you are free to use it (if otherwise legal)
It sounds like you need to familiarize yourself with CA Assault Weapon Laws as it pertains to Handguns.
The easiest method to determine if a particular handgun configuration is an Assault Weapon is to follow the CalGuns Flowchart for handguns.
Since 99.9% of all AK-variant pistols 'naturally' have a magazine outside the pistol grip they are Assault Weapons.
However, you can put a mag-lock device on it to make the mag non-detachable.
But, if the fixed magazine holds > 10 rounds it becomes an Assault Weapon once again.
So, in CA you cannot legally have an AK-Variant pistol with a large capacity magazine unless it was registered as an Assault Weapon.
p.s. you now owe a $5 donation each to CalGuns.net & the CalGuns Foundation for providing you with the info you needed.