So, I have two M&P 2.0 Compact 3.6" slides. As I was looking at the rear sight on one of them, getting ready to replace it, I noticed the set screw sure seemed big; bigger than what I was expecting. Grabbed my other slide, and sure enough. Both factory, both unmolested, but the set screw on the right is significantly bigger than the one on the left. Interesting. I thought all the factory, non-night sight rears were the same. Live and learn.
EDIT TO ADD:
Well, heck. This picture made me look at my front sights, and I see those are different, too.
P.S. Disregard the schmootz on the slide. Came from a linty rag I was using.