Thanks for posting thost photos, helps me understand how they are made.  
The "tail" of the thumbring then lies along the pad of the thumb, right?  I might have to try this with the Kassai horsebow I have been shooting three finger style.  Be interesting to see how long it takes before I can hit a barn from the inside!!!  Plenty of re-learning there.  Maybe I'll shoot left handed with thumbring and righty with other bows,, just to keep them straight.