Gen 1 guns were a pretty slippery, pebble texture, no finger grooves
Gen 2 had a texture like the Gen 3, but no finger grooves
Gen 3 added the grooves and rail (there were also some 2.5s which had the no rail, but grooves...worst of both worlds, nice on Glock)
Gen 3 RTF2s have a much more aggressive texture and some have oddball cocking serrations on the slide
Gen 4s have the interchangeable backstraps.
Incidentally there are also RTF1s and RTF3s...but you will never see one.
Gen 1 guns were a pretty slippery, pebble texture, no finger grooves
Gen 2 had a texture like the Gen 3, but no finger grooves
Gen 3 added the grooves and rail (there were also some 2.5s which had the no rail, but grooves...worst of both worlds, nice on Glock)
Gen 3 RTF2s have a much more aggressive texture and some have oddball cocking serrations on the slide
Gen 4s have the interchangeable backstraps.
Incidentally there are also RTF1s and RTF3s...but you will never see one.