It's a Nikon lens made a long time ago. It's the 25-50mm f4.0. I saw it sitting on the used shelf over at Precision Camera and I haggled on the price until it didn't make sense any more to leave it on the shelf. On a full frame camera it's ---- a 25-50mm equivalent (snicker).
Last night I found an interesting article about this lens and quite a few other Nikon lenses at a site I have never visited before. You might enjoy the articles there; especially if you are inclined to appreciate and enjoy some of the classic, older, Nikon manual focus lenses. The articles I found are on a Nikon website, tucked into a part of the site called, "Nikkor."
Here's the article I read about the 25-50mm lens: http://www.nikkor.com/story/0046/ The discussion of the design parameters and the process of creating the lens are very interesting. Even more interesting is just how good this lens is and how well it stands the tests of time.
I'd love to link to some retail website so you can order one and I can get a commission but, as you can see in the article, they stopped making this one (and a few other favorites of mine) a long time ago (1981).
We'll just have to enjoy the reading and keep our eyes peeled for mint examples, out in the wild...
There are articles at the site on several of my other favorite Nikon lenses; including: the 105mm f2.5 ai and the 135mm f2.0 ai. If you are a dyed in the wool Canon shooter you can ignore this.
Kirk's Books on Amazonhttp://amzn.to/1IYPzXc