Depending upon when you bought your M&P40, you may have an extended slide stop. The difference is that the original slide stop was darn near flat. The extended stop protrudes somewhat more from the side, rather than toward the rear of the pistol. May I point out that the factory slide stop is ambidextrous, there's nothing to prevent you from using your trigger finger on the slide stop instead of the thumb.

Slide stops that extend back toward the rear of the pistol are more prone to contact with the hand(s) resulting in operator induced malfunctions. This isn't a real good situation.

Brownells is back ordered on the factory (extended-it became the standard) slide stop if you want to order one to install yourself.