From : http://opencarry.mywowbb.com/attachment.php?id=2487
Case Law (People v. Hale 1974) regards a concealed magazine as the equivalent of a concealed firearm; it’s an "essential component of the firearm". A full magazine in a holder on a belt should be the same as a firearm carried openly in a belt holster. A speed loader isn’t part of a firearm; you can carry a speed loader concealed in your pocket without worry of prosecution.
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's currently case law. 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.