Sterling Silver cool and dainty Phillips Screw Ring.
Top Diameter : approx. 6 mm
Band width : approx. 2.5 mm
Last pic : same same but with a brass bolt.
If you want to buy 2 or more, check out the stackable rings :
https://www.galbarash.com/product-page/dainty-kicking-mix-dainty-gear-rings-stackable