Is the mag release plastic/polymer? If so, I suspect combination of metal mags and plastic mag release, and a lot of wear and tear/mag dropping/re-insert on the demo gun.
Thanks for the info, I just ordered my first M&P.
It has a metal lip in the polymer. Or blade, if you wish. That was worn way down on mine, but like I said, mine is a 2008 model that was a rep demo gun. It's seen a LOT of wear and tear.
I am betting I am good for another 10k or more rounds.