I checked the SunTour museum site and didn't find this derailleur. But it looks to me as if it must attach somehow to the chainstay just in front of the gear cluster. I know SunTour made such a thing, but I can't remember the name of the model, and I had the impression it had at least some plastic, and this one doesn't seem to.
The first pic shows something engraved under "Suntour," but I can't tell what it is.
(I can't even tell if this is pre-1985, so apologies if I'm straying from vintage territory. . . .)
I'd like to know what it is, that's for sure!
Nath one-could-do-worse-than-be-a-shifter-of-SunTours Dresser