by dtapper2 on Tue Mar 18, 2014 3:04 pm
It's the third cross pin. This one retains the locking block for added rigidity. Since 1990, all non 9mm Glocks came with this third pin and a larger locking block, now all have them. I wouldn't necessarily recommend firing without the pin, but I'm sure it wouldn't be catastrophic. The pin below it also retains the locking block. Without the top pin, you run the risk of the slide lock lever not being under spring tension as that pin is what adds the spring tension on the new glocks.
If you want a guarantee, buy a toaster.