I'm thinking that is actually a No. 4 - perhaps a Parker Hale Custom? Since the rear "ears" for the rear sight are gone, you are looking at a side mount. I've used the Parker Hale A29 side mount, and a 93 (?) weaver flat base rail - drill the rail to match the #6 holes in the A29. Since 75% of the A29 top holes are stripped, I usually drill and tap them to #8x40 as a matter of course. Lets you use weaver style rings. PM me if you want one of mine - i believe I have one left. And, believe it or not, in the early 1970's I actually hand filed a piece of .100" angle iron and made my own side mount base for a #4. Last I heard that rifle and scope are still riding on a snowmobile in Alberta Rockies.