I have a Glock 17 Gen 4 and it says Gen 4 on the left side of the slide. My friend just got a "good" deal on a used Glock 17 but he doesn't know anything about the gun. How can I tell what Generation it is? It does not say Gen 4 on the side of the slide. The recoil spring is smaller in diameter than my Gen 4 G17. My Gen 4 has the double recoil spring and my friend's G17 has a smaller single recoil spring. I can't tell if my friend's G17 has the Gen 4 mag release you can move to the right or left side or if his G17 will accept different back straps.