These are the generations of Glocks as I understand them...
1st Gen- Glock 17s only, had the pebble-grain pattern that is on the sides of the grips of current production guns all the way around the grip; no checkering, no finger grooves/rail. Had one of these and traded it off.
2nd Gen- all 17, 19, 20, 21, 22, 23, 24 models produced until they started adding the finger grooves and light rail; checkering on the front and backstraps, and the half moon cutout at the bottom of the frontstrap.
2.5 Gen- includes Models 21 and 20 that have the finger grooves but no light rails and models 26, 27 that have finger grooves that are not checkered ( I have seen and owned these, and they came from the factory this way)
3rd Gen- Current production, may come with the built in lock-out safety soon
Mino