I can confirm it is for Bayonet 1 TLR - I fitted it to a Rollei. It mates to the inner fitting of Bay-1. It says 180-degree fisheye on it so from that one can calculate the effective focal length. One dial on the lens sets the focal length of the lens it is used on (75mm for most Bay-1 TLR's), the other adjustment is the aperture setting. The adjustable focal length setting is due to the fact that this optic was sold with various adapters for a variety of cameras and lenses.
With a 75mm Bay-1 TLR the iris settings are from F8 to about F32. I've not used it, but I viewed the images on the screen of a Rollei (a Rolleicord or Rolleiflex, I can't remember). It focuses and illuminates some significant portion of the FOV, but not all of it.
I would certainly keep it and use it except for the fact that I now own a Bay-3 TLR (The Rolleiflex 2.8C).
It's a novelty for sure, and not perfect, but it's about $7500 cheaper than a 30mm Hasselblad Distagon
