I'm not even sure that markings on high-caps made for the military or the police during the so-called ban apply to citizens now, anyways.
Police routinely trade in their firearms. In fact, lots of guys look for firearms previously owned by LEOs because they are "carried a lot and shot a little" and have some very competitive pricing. Some pistols come with numerous spare magazines.
In fact, of the bushel-basketsful of magazines I own (going back to the Vietnam war) I haven't a clue if any have markings, nor have I ever seen any.