S&W has been marking new M&P mag followers with some type of marker, making a lengthwise line down the middle, for some time. I've seen light followers get a dark line, and dark followers get a light line. Of course, this might change at any time if the vendor or the company decide to change things. Revisions happen all the time.
Last I heard in one of my M&P armorer update classes, they still test-fire new M&P's using designated test mags, not the mags shipped with the guns.
Last I heard in one of my M&P armorer update classes, they still test-fire new M&P's using designated test mags, not the mags shipped with the guns.