I just got my Model 64-5 DAO revolver from J&G Sales (Serial BET 16XX). I removed the hammer and replaced it with one I had on hand. The hammer I put in is a case colored K frame target hammer. With only this change in hammers, the gun now works in both single and double action.

I think this 64-5 came from the S&W Factory as a DAO model as the original hammer does not appear to have been modified in any way. It also looks like a hammer change is all that is needed to convert back to single and double action.

Hope this helps.

Steve