There have been a report of the slide stop notch peening, and no one has seem to have a solution for. And I've read some wild guesses as to why the notch is getting peen at the very corner. To this, I will offer my theory.
The notch is NOT being peen by the slide locking on the last round. It's being peen due to normal firing and every time the slide goes rearward and the stop is BOUNCING and kissing the notch. The reason the slide is not looking back is from the lack of an empty mag and follower to keep it in place.
So the culprits behind this could be a couple of reasons:
1) a weak plunger spring.
I've taken mine apart and the stock spring is as strong as a wet noodle. It was a joke. So I replaced the whole plunger spring and ends with Ed Brown replacement kit. I also polished both ball ends with some fine grit sand paper for a smoother finish.
After installing the new spring and caps, the slide stop wouldn't engage on an empty mag, which means that the new plunger spring was just a little too strong for the mag spring. I had to manually engage the stop upward. I removed the slide stop and saw the marking from the ball end. I took some used 600 grit paper, wrapped it around a small screw driver, and polished that area on the slide stop. Insert the stop and it worked on an empty mag.
So, the stronger plunger spring will keep the stop from bouncing, during firing, and will properly lock the slide on an empty mag.
2) hand grip.
Depending on your grip, it can come into slight contact with the SS and cause it to move slightly up, thus making contact with the notch, as the slide goes rearward.
So there are my theories, and I'm banking on number one, and I hope my "solution" will work out. Only time will tell.
Here's a picture to better help illustrate what I'm talking about.
Here's the back side of the GSG slide stop. Note the scratch or contact area of the ball end of the plunger spring. You will also notice the two slopes; top and bottom, on this back side. The top slope should help to keep a downward force on the SS from bouncing up, during normal firing. And the bottom slope should help to keep the SS up, once the mag follower has engage the SS.
You'll need to polish both of these planes for a smooth, low friction, travel for the SS to work with a true 1911 plunger spring that replaced the stock GSG plunger spring.
Went to the range and put another 200 rounds thru her. The modified SS and new plunger spring works perfectly. And the slide doesn't show any signs of peening.