T-mount on one side and something strange (similar to Arri PL? on the other)?
Since it doesn't have any markings I doubt its a lens for any consumer camera. I thought it might have been a DIY type 3D printed lens, but the finish is way better than what I've seen come out of most 3D printers.
My top two guesses
1) Lens for a rifle scope (with some kind of eye pad attached to the other end)
2) lens for night vision binoculars. link