Originally Posted by Sevens
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
That piece is the sideplate assembly and it is present on 2nd
generation guns. If you look at the shape of the part in the picture of the 2nd generation gun, you will see that S&W simply made the grips for the 3rd generation guns to barely cover the part; you can see the edge of the part hiding just behind the grips in the picture of the 3rd generation gun.
One of the more prominent differences between 2nd and 3rd generation guns is that 2nd generation guns have straight barrels and 3rd generation guns have barrels with a smaller "waist" near the end.