From the FAQ at http://www.californiaopencarry.org/faq.html
I've heard concealed magazines are illegal. Is that true?
While there is no code that supports this, there was an appellate case in 1974, People v. Hale, that ruled this way. It ruled that although the firearm in question was not concealed, the magazine was, and that only partial concealment is still concealment, and that the magazine was an "essential component" of the firearm. The logic is tortured beyond belief, but it currently could be used as a persuasive precedent in court. One way to avoid this pitfall is to carry your magazines in belt holsters, so that they match the 12025(f) language of "carried openly in belt holsters". Another option would be to keep an unloaded magazine in the firearm, thereby "completing" the firearm, and invalidating the asinine "essential component" logic.