I found this forum looking for the same answer. Took possession of my Gen 1 glock .40 yesterday after my 10 day wait. Bought it used off a buddy back east and sent him the parts to convert it to be CA compliant. While I was doing my 10 day wait I got sucked into reading threads and said screw it, it looks like I'll have to tweak the gen 2 rail to make it fit, so I ordered one. Picked up the gun yesterday and that was the first thing I did, it fit perfectly, there's no gap like the picture shows where the guy put a credit card in it. I did have to take a round file to the front picatinny to clear the rib on the catch when you close it. But yes it fits fine.