The part you're referring to is the Locking Block. http://smith-wessonforum.com/smith-wesson-m-p-pistols/341115-m-p-parts-identification.html
The "wear" you see isn't wear at all. It's a casting mark from the manufacturing process. Some have it and some don't. My .40 has a similar mark, but my .45 doesn't.
It is nothing to worry about.
calm down and shoot on.
I completely understand. I probably would have thought the same thing. I've just seen a bunch of M&Ps now and you're not the first to ask this question.My shield has over 1k round thru it and it doesn't have it so seeing that for the first time freaked me out a bit.