This is the second 16mm fisheye lens sigma makes. They visibly dropped the YS interchangeable mount but at a closer inspection, the whole lens has changed. Focusing and aperture rings now have diamond/shaped rubber. The Filtermatic ring now is moved toward the lens hood and not in between focus and aperture. The whole lens is a bit shorter. Please note the elaborate Nikon F – Ai coupling ring.
This is a rare find, Lens as new in box with its own beautifully engraved and preserved quiver, documents etc.
The inner part of the cap ring is flocked like the hood.
Please note some clear differences from the previous model, including the velvet flocked inner part of the hood, which has now a simpler shape without the step. The front cap diameter is the same. This, or its next incarnation, is the lens you always see from distance in the cabinet of historical Sigma first lenses. It was probably sold also under other names, but I did not find any proof yet.
Here is a nice comparison of the lens family evolution, from left to right YS, XQ, and the last filtermatic.