20 mm diameter x 2.8 mm x 2.0 mm width / Brass 2 g / $ 2
25 mm diameter x 3.2 mm x 2.5 mm width / Brass 3.5 g / $ 2
30 mm diameter x 3.2 mm x 2.5 mm width / Brass 4.5 g / $ 3

This is a quality solid brass split ring.
 
This ring is made from a brass wire bar by skilled Japanese craftsmen.
They made it by bending a brass wire rather than pressing it.
 
If you put a key through direct to a split ring,
2.5mm ~ 4.5mm in diameter of the hole of the key is necessary. 
 
Material / Solid brass
 
Color / Raw brass