Found this today and found it interesting.

http://www.primitiveways.com/archer%27s_thumb_ring.html

Very interesting...... I always wondered how those worked. I don't know about resting the arrow on the opposite side, that would really mess ME up.

Those are thumb rings....not thumb rests.  They have been, and are used, by many archers...today they are still used in recreative shoots like the SCA get togethers, and many folks who shoot the short horse bows.

That is a weird one.. Why would you need a loop to use a thumbring. The ones in that website is way too long also.

These are ones that I made. Not very pretty but they are functional: