Looking at an older Gen2 gun yesterday and comparing to a gen 4, other than some grip texturing I can see no major differences in the guns, they sure do not seem to have made any mechanism changes.
I don't remember all the specifics, but for a serious defensive pistol, I wouldn't recommend a 1st or 2nd generation Glock.
Around 1991-92 Glock came out with the "6 part upgrade"; which should have been a recall, because Glocks were very problematic until this upgrade happened.
Extractor
Spring-loaded bearing
Firing pin safety and spring
Firing Pin
Striker
Trigger Bar
Anyone who knows anything about Glocks can tell you these parts comprise the heart of the Glock, so that was a major freaking upgrade. It also means that the gun in 1st & 2nd Gen was fundamentally flawed.
When Glocks started turning up in IPSC competition in the late '80's & early '90's, I've never seen a firearm fail as much (although it was rare to see a G17 fail).
Even after the 6 part upgrade, the next step is when Glock was trying to become the new pistol for the Secret Service and it failed the "Frisbee" test. Simulating an agent running and taking a face plant, the pistol was thrown like a Frisbee onto concrete, and the Glock's slide consistently popped off the frame.
By Gen3, all of these issues were corrected and the Glock was an EXTREMELY well made, and reliable pistol.
So while the 1st & 2nd gens are interesting to shoot and collect, it wouldn't be my choice for a serious defensive handgun.
