The shank you describe sounds like the one made by Finger Mate. We
have used most of the options in the market and we feel the Finger
Mate is the best. The mechanism is, as you described, completely
encased inside the shank. They make them in many widths and several
different metals. They are located in Hallandale, Florida.
888-458-2700. The inventor of the shank just pasted away and this
might explain them not getting back to you. I have to warn you that
they are one of the quirkiest companies to deal with. Their product
is excellent but somewhat difficult to install. We do install them
but, if time allows, I prefer to send the ring to them for the work.
If they do the work you have less trouble with the warranty. You can
request, for a fee, a sample ring.