The Nikkor-O 21mm f4 is an ultra-wide-angle lens made by Nippon Kogaku for the Nikon S series. It was introduced in 1959 and features a 4-group, 8-element construction. The serial number is 621xxx, with the first three digits being non-sequential, so-called prefix numbers. The distance scale is marked in meters.
While the Nikkor 21mm f4 for the Nikon F mount is relatively common, this version for the Nikon S series is extremely rare, with only about 298 units said to have been produced.
The lens comes with its dedicated viewfinder, which has a 5-group, 5-element construction. The viewfinder’s serial number is 600xxx. Made of plastic, it is lightweight. Originally, the viewfinder was accompanied by a deep rear cap designed specifically for it, and it also features a mount for securing the viewfinder. This is a rare item.

This lens appears to have been used quite a bit in the past, with small scratches, scuffs, and some paint wear on the barrel. There is also some wear on the paint around the protruding rear element, but no noticeable large scratches or dents.
The front lens element shows some fine cleaning marks and coating wear, but during the test shots, it didn't seem to significantly affect performance. There is no fogging present.
The included viewfinder shows some paint loss and linear scratches around the eyepiece area. The plastic parts have age-related wear with some glossiness, but the glass is free of scratches and in good condition, providing excellent image quality.
The test shots came out very well. I would be happy if you could take a look at the sample shots in the gallery. There is noticeable falloff at the edges, but the center of the image is exceptionally sharp.
Mounted on a well-used Nikon S2, SP, or S3, it would look quite appealing. It’s a rare and enjoyable lens to both view and use.
The lens comes with front and rear caps, and a filter. The filter has one dent on it, and the rear cap is a third-party item.