The second pin in this area of the frame was added to stabilize and reinforce a new, larger, locking block in the frame. I believe this was adopted due to the older .40 caliber Glocks having a minor problem with what is commonly called "peening", where the older/smaller locking block would bounce upward as the frame flexed in recoil, allowing the top of the block to strike the bottom of the slide. This would leave small marks on the bottom of the slide, looking like it had been struck with a small metal hammer.
With the larger block and a second pin to hold and stabilize the block in its frame channel, this problem is much more rare, and very limited when it does occur.
In 9mm, the older style locking block and single pin worked just fine, and I don't think you'll have any problems with your Glock. I had one like that myself a while back, and I shot and carried it regularly for more than 10 years with absolutely no problems.
Last edited by DJ Niner; 11-10-2012 at 02:34..