hope this helps:
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.
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