It'd be cool if it were modified "native" F-mount, but I bet it's an M42 version with a Nikon adapter attached ... I had one of these lenses with a Pentax K adapter that was so firmly attached it was almost unremovable.

That lens has an interesting look, but you've really got to keep the subject in the center sweet spot. This makes for an awkward composition.

It's also the single most frustrating lens I've ever used. I regret selling mine a bit.