There are two long depressions stamped in the sides of the lower portion of the 13-round magazines that is lacking in the 15-round magazines. If you drill a hole near the upper end of those depressions, with diameter the same as the width of the depressions, and also clip half the length off the two plastic studs on the bottom of the follower, the magazine will hold 14 rounds. I think it might be possible to further modify it to hold 15, but haven't really looked at that yet.
I'm not saying this is a good idea - just interesting.
I'm not saying this is a good idea - just interesting.