This is a Hollywood style movie camera lens that has been converted to take 1 1/4 inch accessories. Made by Birns & Sawyer, the label calls it an 'Omnitar 1:5 500'. Since it was originally a camera lens, it has a built in adjustable aperature going from f/5 to f/22. It also has a slot for drop in filters. The large ring is the focuser. The back end has been modified to take 1 1/4 inch accessories. It comes to focus with a camera or diagonal and eyepiece. The bottom mounting plate is drilled and tapped either 3/8-16 or 1/4-20, whichever you want to use. Makes a great long focus lens for your camera or use it as a 500mm refractor. This thing is 23+ inches long and weighs about nine pounds, no lightweight. Optics are in perfect shape, minor cosmetic dings in the external paint.
Yours for $250.00 plus shipping in CONUS, PayPal add 3%, MO okay.