TexasCHLforum.com

Here is most likely why the State of California wants to ban them out right, since magazines have no serial numbers and no way to prove when they were bought then there is no way to disprove that you didn't have it before the ban, so they are laying down a blanket prohibition!
I have 4 14rd mags for my 1911 bought in the 90's
20 30rd mags for the AR given to me by my nephew when he returned from Afghanistan
one 13 rd and two 29rd mags for my .45 carbine
None have any serial numbers so no way to prove when they were acquired