A friend of a friend has the 6 mm f/5.6 Fisheye-Nikkor. Tried to convince him selling it to me was a good idea, but he didn't take the bait .... Oh well, as it was made in approx. 109 samples, I do have rarer Nikkors than this.
Still, would be interesting to CPU-modify any of them for use on a modern camera.
The 6 (6, 6.2, 6.3)mm class fisheyes are listed as having 220 to 230 degrees field of view.