Nighttrain,
I also have a JC Higgins Model 50 that is missing the rear site, and I have given up looking for the exact replacement, although like 458Win says, he and others may have them in boxes somewhere. It appears most eveyone took the sight off so a scope you clear.
I purchased the one I have as a donor, but made the common mistake of shooting it and now I cannot make myself take the barrel off.
I think some of the aftermarket sights will work, but may need a little work with a file to fit the dovetail correctly. I will look into that over the next few days myself, and if I find something I will get back to you. With my bad eyes I don't need open sights, but I would like to give it to one of my grandchildren and they all see very well.
