Mounts a camera (with your T-ring) to any 1.25" eyepiece holder for prime focus astrophotography. Just attach the adapter to the T-ring, insert into focusser tube and secure. This has only been used twice that I can remember.
This is the same design and appearance as those sold by Meade, Celestron and others. It seems to be a bit heavier than most. Since the Televue logo is on it, it was more expensive than others- they charge about $47 for it. You can see it on the Televue web site.
Asking $ 17 + $3 postage.