Learned some stuff and thought I'd post it here for others with similar questions. I spoke with the CA Dept of Justice firearms division. According to them (confirming what someone said here) I (or any citizen) can get an S12 with non-removeable mag, up to 10 rounds, without department authorization. It is however required that you check with the County DAs office as some countys prohibit bullet buttons.
To get the S12 with detachable mags and in "assault weapon" configuration, I will need to secure a letter from my dept, on letterhead, that is 1) addressed to the gun dealer, 2) identifies me