This compact and nice-looking lens was the first diagonal fisheye produced by Nikon for their 24×36 reflex system, in 1972. It did not require mirror lock-up and could be mounted on every camera of the line. The first version is easily recognizable from the diamond-shaped slim focusing ring.
sn going from 272281 to 175988.
The lens was aesthetically revised in 1976, with the rubber focusing ring now shaped in two lines of rectangular bumps. On the hood the engraving becomes “16mm 1:3.5”. The second version of the lens is the rarest.
Unfortunately, all the 16mm 3.5 lenses you will find on the market are not original state anymore. Nikon offered an upgrade service for all their old lenses, switching the aperture ring, and sometimes the bayonet, to fit the new AI meter coupling. Since Nikkormat FT3 and F2as Nikon changed the metering coupling system, from the ubiquitous “fork and pinion” hold on the aperture ring, to a tooth-and-ring one. Old toothless lenses would not meter anymore, and sometimes they could not even be mounted. All pro and serious amateurs who wanted to keep their lenses value and functionality did it.
A good article in dpreview explains about Nikon bayonet history.