The ring has a anchor with a rope wrapped around.
Sz 7 - 18 mm
Sz 8 - 18,4 mm
Sz 9 - 19,1 mm
Sz 10 - 20,1 mm
Sz 11 - 21,6 mm
Sz 13 - 22,5 mm
The sizes can differ +/- 0,1mm
Material: 100% Inox Steel (316L Stainless steel)
Inox steel has a small amount of nickel in the alloy. It is often called nickel free but if you have a bad case of nickel allergy you should take caution.
Not sure which size you need? Read this about how to measure.