I see what you mean now, the aperture stop-down lever at the rear has a chunk missing from the lower side. I have seen other lenses like this. I'm not sure it is due to wear since your lens does not appear to be heavily used, neither were other lenses I have seen. It is not part of the AI conversion process either since I have seen native AI lenses like this (including my AI 105/2.5). I can only guess that the tab was milled in the factory to calibrate it. With pre-AI and AI lenses the precise position of the tab is not critical, it's basically set to open (for full aperture viewing) or stopped down (as set via the aperture ring at the moment the picture is taken), but maybe some adjustment is needed so it properly engages with the lever in the camera.