Great picture in the first post in this thread:
The little piece I'm always looking for is that small piece of flat metal under the grip panel on the left side (pictured) of the pistol. Maybe it's a piece that is connected to the slide stop retainer?
It's silly that this small piece is what I look for to know instantly that it's not a 3rd Gen, especially with your comprehensive list of differences -- the grip panels being most obvious.
But it's that little piece...it looks out of place, like an afterthought. Not that it's an "ugly mark" or anything, simply that it just looks like something they forgot to "clean up" between final design and production.