Glock 17 early/later gen4 differences - beaver tail notch?

[rob350]

Hey, total glock noob, but getting very interested in a g17 now. In particular gen 4 ( for reversible mag release).
I heard some talk about a early gen 4 that does not properly hold the beaver tail grip attachments on. This true?
How does one differentiate between early gen4 and later gen4 (I would prefer later gen4 for the option). I suspect it's a extra notch or somthing, picture? My Google foo has failed me. Any other differences?

 

[ArcticHound]

If you look right where the beavertail would sit, there is a little rectangular bit that sticks out on the newer Gen 4s. Without this, the beavertail backstraps don't hold properly in this area and can lift away. That little tang locks the backstrap down at that point.

 

[ArcticHound]

This video shows what I'm talking about: