The 92FS has a rear strap indentation. The M9 does not. BUT, b aware that in the past year - there have been some batches of commercial M9s built on 92 frames. And, early this year, I bought a 92FS built on a M9 frame. So, Beretta does some variations - however, the item #'s do not change. Its just some subtle variations - apparently with whatever frames/parts they have on hand.
However, these are items/gun variants you just have to find - the M9 isn't supposed to have that rear strap indentation - and the 92FS is supposed to.
This small grip difference makes a difference for people, like myself, with smaller hands. It brings your top finger ever so closer to the trigger. I can feel the difference, and prefer the 92FS frame with that difference.
here is a pic to show: