If you're talking about the mold marking on the center rear of the locking blocks, they're normal (and vary from one block/mold to the next).
Here's a couple of M&P locking blocks (new & used) which exhibit the mold marks to varying degree. Some are smaller, some are "cleaner" and some look downright nasty.
Cosmetic. Not a problem with the way the blocks are made (according to S&W, and I've been through the M&P pistol armorer class twice).
FWIW, I remember when some owners of Walther P99 pistols first saw the MIM locking blocks (and the corresponding "sprue-type" mold marks) and freaked.
