DuBois -
“I am wanting to try using a thumb ring and not sure if the string rests on the inside of the thumb or is resting in a groove on the ring. Maybe there are some of both type in use. I'm making one from bone and not sure how to proceed.”
Both types of thumb rings are used. Generally the user’s find (but not always) the simpler - the one without a secondary groove - to starts with.
Nota bene : The sized of the ring - the slightly oval - is CRUCIAL .
“Also, is there such a thing as an Index finger ring for the right hand that would allow shooting arrow from left side of bow and give the similar torque as the thumb ring and arrow on the right?”
Not as far I know know . The modern Mongolian (circa the 1930’s) used either hand . . .
(. . . pictures : a selection of the rings + one of my own . . .)
regards,
Scyth