you are buying Traditional bow thumb rings. it was hand made by workmanship. male thumb ring is more easy to pull string and no pain on finger and no slippery. you need to check width of your thumb joint as picture posted and choose your size. if you don't know exact your size, buy smaller one and grind inside by sandpaper or grinder.
Thumb ring sizes: 19mm, 20mm, 21mm, 22mm and 23mm
Material: Genuine Korean cow horn.