Well, if a magazine was stamped "military/law enforcement only" it can be reasonably assumed that it was manufactured in the time period that the federal "assault weapons ban" was in place. The ban expired in 2004, 4 years after the cutoff date for legally purchasing a "high-capacity" magazine in California, putting the "military/law enforcement only" magazines on market. I'm not a lawyer, but I'm guessing that because you could only practically obtain Mil/LE-stamped after 2004, it would be easy to prove the guilt of the party in possion.