Here's a couple of M&P locking blocks (.45 on the left and .40 on the right). You can see the mold marks on each at the center, although they vary in appearance. The .40 LB is used, but the mold mark visible was there from when it was made.
You can probably call and ask to speak to tech about it if it would help ease your mind. If there's any question in their mind they'll send you a shipping label for free shipping under their lifetime warranty. Call them if it concerns you.
BTW, last time I asked, each new M&P is test-fired with 3 test mags loaded with 5 rounds each, for a total of 15 rounds.