Nikon 500mm for Pentax 6x7

Hello-

I have a lens that seems to be very rare, everyone I have spoken to over the phone about this lens thinks I am crazy (maybe I am). Anyway I am trying to find out some information about this lens. Here are the specs-
Nikon Lens (Reflex Nikkor)
500mm mirrored lens 1:8
Mount is for a Petax 6x7 (and yes it works on the pentax)
Lens cap is leather and says Nikon on the cap, on the side of the cap it says 91k made in Japan
Basically it looks like my other 6x7 lenses, barrel size and all.

My guess is it's about 30+ years old. It came with the Pentax 6x7 package I bought 15 years ago and the body was the first generation of 6x7's. I'm at a point where I'm thinking of selling this because I've only used it a few times over the years.

If anyone has any ideas or information about this lens I would love to hear some feedback. Thanks in advance for your comments-
Alexis : : : :