I saw your suggestion of the Osgear and have to admit, with apologies, that I didn't think about it seriously. I'm not familiar with all the Osgear variants.
The frame is a bit more like an archaeological site than usual. These cable stops and guides are probably original. The shifter bosses, whatever they are, are probably not original. I'd like to look over every inch of the frame and see I'd there are more signs of changes made.
Returning to the Osgear, I guess this would be the Cyclo Ace or the like? I can't find complete photos of that system. Was it sometimes mounted on a brazed on tab? What shifter(s)?
No apologies necessary, my posts are usually short and I probably leave out relevant details.
My knowledge of Osgear and Osgear-like derailleurs is very basic at best.
My first guess for a derailleur setup on the OP's frame would be one of the better Osgear units with the adjustable tension arm cable, and yes, some Osgears were mounted to brazed on tabs.
The shifter(s) were probably a right side mounted double levered unit.