I have a couple 5th gear syncros with two brass syncro rings, one ring for the 5th gear position and one for the reverse.
I also, have this one (photo attached) that has a brass ring for 5th gear position and no brass syncro ring for the reverse position.
The syncro in the photo has a much heavier hub and brass syncro ring than the others. I don't know why a syncro ring is need for reverse.

Can anyone identify it? When was it used? On what models and years?

Looks like this one is much better and stronger that the others.