All the Gen 4 19s I've seen now come with the two extra beaver tail backstraps and the little notch at the top rear of the frame for fitting the beaver tail backstraps. I haven't checked on any of the subcompacts yet.
My Gen 4 Glock 34 and a Friend's Gen 4 Glock 17 have the square notches but did not come with the beavertail
Interesting. It looks like they changed the frame before they added the backstrap to the box. All I know is my friend bough his Gen 4 19 near the end of last year and his doesn't have the notch. Heck, I didn't even notice the notch on the newer ones until he pointed it out to me yesterday; I just thought the only difference was that they shipped with the extra beavertail backstraps.