How does one prove that the mags in their possession were purchased/owned prior to ban?
The burden is on the state to prove you acquired them post-ban. The previous ban started in 2000 so anything obviously made after that time was pretty easy to prove.
I would think that this period when the entire section of the California Penal Code was enjoined just threw that out of the window for any current residents there.
Of course it remains to be seen whether or not California will get to reinstate its ban on mere possession.