Just upload this picture to Flicker so you may see what that part looks like on three of my M&P's. This mark is easily seen on the 40, and not so much on the other two. From left to right, 9L, 40FS, 45FS,
JJEH- I have about a thousand rounds through it. Just shot it for a few hours so when I get it cleaned up tonight I'll take another look and take another picture to see if there's any change.
I saw a test where the M&P they were abusing (no cleaning, no oiling, rapid fire) broke down in only 65,000 rounds with a cracked frame.... S&W examined it and gave them the go ahead to continue using it as it wasn't that bad, but they stopped the test.
Probably because they couldn't hear anymore and their arms were like jelly.