You are correct that simple posession is not regulated in CA.
Unfortunately your last statement is incorrect. The simple act of bring a large-cap mag from a state other then CA into California is importation (and illegal unless you qualify for one of the exemptions)
The most common exemption is that it is legal to re-import large-cap magazines in CA if you posessed them within CA prior to 1/1/00. Residency nor Ownership are an aspect of that exemption.
For citation: This is all covered in CA Penal Code 12020