This is a quality solid brass split ring.
 
This ring is made from a brass flat bar by skilled Japanese craftsmen.
They made it by bending a flat bar rather than pressing it.
 
If you put a key through direct to a split ring,
3.5mm ~ 4.5mm in diameter of the hole of the key is necessary. 
 
Material / Solid brass
 
Color / Matte black plated

Size / 27 mm diameter x 2.8 mm x 3 mm width

Weight / 5.5 g