Bill (xausa) is right, that is a base for a Lyman 35 sight. A few years ago, I got a parts grouping with an unfinished base. In the midst of discussing rifles with Bill, I learned that he had a base on a rifle but no slide. He agreed to send the base to me so that I could study it, in return, I said that I would make a slide blank for it and if possible, make a slide. I was able to make a slide for his and complete mine as well. The slide was not an exact replica but was functional. Here is a link to some of the work I've done.
Lyman 35/36 stuff I've got some slide blanks lying around and I could make another slide like the ones I made for Bill and myself, but I would need to have the base for a while. There were some small variations in the Lyman 35's from what I have seen, and I would need to do some hand fitting of the slide to get it to work with your base.
