Just looked at my 4506 and my 4046 and oddly enough the bent over end of the sideplate assembly on either of mine does not cover any part of the hole for the slide stop internal end. The part I am referring to is number 66 on the exploded diagram available on the Numrich website. If I am looking at your pictures correctly, it appears that the hole for your slide stop is partly blocked by this part and may require a replacement ($12).

It also appears from your blurry photos that the nub that catches the notch in the slide is worn away on the inside. That nub on both of my slide stops actually extends inward into the notch in the side of the frame. Yours looks like it is worn even with the edge of the slide, and if so any lateral play will cause the nub on the slide stop to miss the notch in the slide. But I'm not going to take odds on this because I can't see it clearly enough in the photo.

I'm sure someone with vastly more gunsmithing experience than I will chime in and verify if this is an issue.