That 'dingus' is the magazine disconnector. It's kept in place by the rear sight. Since you've removed everything else from the slide, you might as well remove the sight, too. As explained to me by Model 39, when I queried on this site a few days ago (Model 39 Rear Sight Removal), you pry up the long spring at the front of the sight. There's a notch in the slide for this. Don't pry too much, as the spring can break. Then pivot the sight 90º either way and lift it off. The dingus and dingus spring will come out too. When I did it, the amount of crud under the sight was amazing; I wouldn't doubt that yours is in similiar condition..