A transom is a single pane hinged window frequently found over old doors to allow air circulation. The ring is to allow a pull down hook to release them but an option would be to attach some sort of rope or chain. If used within easy reach, a finger fits nicely in the round hole!
Constructed of heavy cast brass (or bronze) they remain in excellent condition with strong springs. They have been cleaned but retain a nice warm patina. There is no catch provided but you should be able to find a strike plate that would work nicely at your local hardware store.
If you need a Pull Down Hook you can find one at WO1019
Latch: 1-3/8” x 2-1/4”