Hi everyone,
I'm glad I found this place - seems to be a lot of knowledge around Nikon here.
I use a couple of Nikon lenses regularly, however mostly the very experimental types (enlarging, scanner and industrial lenses).
During my research fo an article on Tomioka which I'm currently working on, I've stumbled upon a lens (very likely made by Tomioka), which has the following inscription:
N.J.K. M=Tominon F4 60mm №1000030 
As you can see it's a 60 mm lens and in pretty bad shape. My suspicion is, that it was a prototype lens for a Yashica macro lens... but that's besides the point.
The main reason I would love to hear the opinions of some of the experienced Nikon people here has to do with this:
NKJ is an abbreviation for "Nippon Kogaku Japan", your lens will have this engraving on the name ring inside the filter ring. This engraving is found on Nikon lenses up to about 1970 when NKJ was replaced by "Nikon".
So apparently, Nikon used a very similar abbreviation on some of its earlier lenses. While that could be seen as a mere coincidence, I got the information from a person who worked with japanese optics industry for a significant amount of time that he has seen N.K.J. on Nikon enlarging lenses as export products, but that he has also occasionally seen an N.J.K. engraving on a Nikon lens. Now I don't know of any cooperation between Nikon and Tomioka, but with the Japanese optics industry everything seems possible... so I would be really grateful if you could help me answer a couple of questions:
1.) How does the NKJ on Nippon Kogaku lenses usually look like? Are there full stops behind the letters. Is it in color or white/like the rest of the inscription? Does anyone here have some images of lenses like that?
2.) Have you every heard of or seen an N.J.K. inscription on a Nikon lens? If so, which one?
3.) The lens in question is a 5/3 Xenotar design (very similar to the Micro-Nikkor 55 mm f/3.5, if I'm not mistaken). Has there ever been any rumor about Nikon working together with someone else on one of the early Micro-Nikkors with a design like that? (I suspect the lens might be from the late 1950s/early 1960s)
4.) If we assume that Nikon usually only used N.J.K. on its lenses as a unique marker, can you think of any other possible manufacturers who would therefore have to change the letters up to have a unique marker as well? I was thinking of Nitto Kogaku... Anything else?
Thanks a lot in advance!
