What I do see in the first picture, the top one, is what appears to be welding metal or welding flash on top of the frame below the raised sight that appears to be blocking the sight from moving from below the 300 yd setting.
It's not actually a piece of weld, the best way I can describe it is, its basically a little circular metal piece about 3/16" to 1/4" round with a hole drilled into it the same size as the hole in the front of the slide. It appears it was hammered into the slide.