Back ~1960 my Father purchased a Marlin 39A, took off the "buckhorn" sight and installed a Lyman receiver sight. Then he replaced the usual fixed peep with a Merit Adjustable Aperture. They were popular for target shooting, but there was also a smaller version ideal for squirrel hunting. I still have that rifle, can't ever get rid of it, but can't even begin to think of putting a scope on it even if it would be more useful. To me, it's better to just purchase something else and not mess with perfection.
If you can still find them the threads seemed to match most if not all of the other peep sights at the time.
From the looks of it they may be out of business, but at the bottom left of this page is a very good picture of what I'm talking about.