Some Norinco M14 mags have a skirt on the bottom of the follower that restricts them to 5 rounds (from 20). The real test for mags is this, take a mag and six appropriate rounds, if they all fit it's illegal. If you have one left over, it's
not. The CFC made reference to this in their recent letter to Demented, something along the lines that since it's illegal to convert them they would accept a rivet, even though it's reversible. If an officer wants to have a problem with you they will, there's nothing you can do about it - just see them in court.