The used 4013 I bought & converted to a 1013 had that same mark & mine appears deeper than your's. One of my other 3rd Gens (4516?) has it to a lesser degree also. Suspect it's more likely to happen on the aluminum frames.
As I recall, the back side of the slide stop lever had a protrusion manufactured into it at that point, instead of being flat. I don't have a good picture of it in my file but it was not the plunger retaining pin in my case. If I get a chance I'll post a picture of it.
.