This is (I believe) the second version of the Joe's derailleur, released around 1998. The 1st generation had a much more rounded cage design, among other changes. Prototype Machining was Joe Glader of Denver's company. This derailleur is often confused for the Rhino, and they share many design features. The cages and pulleys are nearly identical. It is also quite similar to the Paul Powerglide derailleur.
These derailleurs are EXTREMELY rare. They almost never pop up on the market. They're much more scarce than the Paul RD, and a bit more than the Rhino.
*This derailleur is missing the mounting bolt. It has the sleeve that fits inside the body, just no bolt. It doesn't appear to fit a standard derailleur bolt as is, since the sleeve sticks out the back of the derailleur about 2mm creating play. I do not know if this is how the sleeve is supposed to be, or if it used a special bolt. If anyone could enlighten me, I would be much obliged!
Very good to great used condition. Pivots and springs are tight and smooth. Jockey wheels are super smooth, with minimal tooth wear. The aluminum surface has some light marks throughout. It's an unpolished finish, so this could be buffed out to look near new with a bit of love.