Boxhead,
I am guessing that the fellow you sold the revolver to had a scope mounted on it, correct?
If the Gunsmith that did the work failed to return the original rear sight, perhaps your friend recalls who did the work so you might give the Gunsmith a call and see if the sight is still in his parts inventory. I know it is a long shot
steamloco76 you must be looking in the wrong places.
After reading your post, I went and did a search on eBay. In my 1st search, the 1st page of a 4 page result came up with 7 old style square ended tang Smith & Wesson rear sight assemblies for sale. They were priced from $55 and up
Now I did not read the body of the offerings to see what they came off of, but they are out there and thanks to the Internet they are easy to find.