As an example, the Glock 43 was never around back then. So while it is the first Glock 43, it is still considered in Glock's 4th generation pistol line. It wouldn't be a Gen 1 Glock 43.
Same with the Glock 26. It was never around when the 1st generation pistols were. Going off memory, it was introduced in the middle of gen 2 and gen 3, and the first pistols were what are called gen 2.5 pistols, as they had traits of gen 2 and gen 3 pistols, but were not full fledged gen 3 pistols. That's how I remember it at least, and I stand to be corrected if any die hard Glock guys are around.
I found this quick quote on the interwebs:
- 1st Generation has the smooth, rounded “pebble-grip”.
- 2nd Generation has the “Grenade-style” checkering with NO fingergrooves (see picture).
- 3rd Generation has BOTH fingergrooves AND accessory rail on the front dustcover.
- If you have a subcompact (G-26, -27, -28, -29, -30, -33, -36) OR a “transition model” when Glock was putting fingergrooves but no accessory rail, then you have a 2.5 Generation. There is even a slight variation here, as some of the earliest G-26 & G-27s had smooth fingergrooves, without checkering in-between the grooves. Some people do not count the sub-compacts as 2.5 Gen since they came out after the 3rd Gen frames were already being made. Note: newer -29, -30 are now coming with rails.