For the 9mm/.40/.357 full-size pistols, a close look at the frame grip will tell you the generation:

For Gen4 and later, they stamp the Generation (Gen4, Gen5) on the left side of the slide. Gen5 models have a similar gripping pattern to the gen4, but without any fingergrooves.

Gen1 and Gen2, mechanism is virtually the same. Starting with Gen3, they added the weapons-light rail on the dustcover portion of the frame, but most parts would still interchange with Gen1 and Gen2 (with a few exceptions, such as locking blocks). Some Gen3 pistols will have 2 frame pins like the Gen1 and Gen2 (one at rear of grip, one over trigger); other, later models will have three (adding the second smaller pin over the trigger, called the locking block pin). Starting with Gen4, many more internal changes, including new-style dual-spring recoil spring assembly, reversible magazine catch for left-handers, and modular changeable backstraps to add size (and beavertails) to the basic grip. Much of the internal mechanism was the same, but small tweaks required slightly different parts. Gen5 is darn near a whole new animal, many major internal changes, different barrel rifling, ambidextrous slide stop lever, different springs in several areas, back to 2 frame pins, back to no fingergrooves.

This is not an all-encompassing list, but it hits most of the high points. Larger frame/caliber Glocks did NOT get the same changes at the same time, nor did the subcompact 9mms/40s (see below), so don't apply this photo or list to other frame sizes/calibers.

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The 9mm/.40/.357 subcompacts are a bit different, as they haven't been around that long:

The main difference between the Gen2.5 and Gen3 in the subcompacts is the fingergroove area of the frame; Gen2.5 is smooth, Gen3 has checkering in the fingergrooves (hard to see in the photo). Gen2.5-Gen4 use the same recoil spring assembly; Gen5 is different (I think). Gen5 models have a similar gripping pattern to the gen4, but without any fingergrooves, and have many internal changes (same as the larger-framed Gen5 models).

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Is there a weight difference between these ?

Someone would like to know what I know the trigger housing mechanism between some of them are different between the generation three and the generation four.

 

Yes, the weights between calibers vary, as I believe Glock makes the slide assemblies on the more powerful .40 and .357 pistols heavier to reduce the slide speed. The best place to go for comparative weight specs is Glock's website.

As for the trigger housings, you'd have to talk to a Glock armorer that has some experience working on each model/generation, and a current parts list. If you ask in the General Glocking forum here, a current armorer will probably be able to answer your question (I'm not sure how many Armorers we have reading/browsing in the Collector's Club).

 

Some also call frames that have Gen3 finger grooves but no accessory rail Gen2.5. Other than subcompact models and the 19c these were all large frame models G20, 21 30, 36 ect along with compensated models.

There's too many little variations to label them all, even with the Gen5s Glock has already removed the notch in the front of the grip in the G19 with the new 19 MOS that just released.

 

Wow, DLNiner, thanks for the most excellent explanation visual I"ve seen! That was extremely helpful.