I found an Old Model Single Six a week ago. It has a 6 1/2" barrel and and aluminum frame. According to Ruger's web site, it was made in 1970. It's not been modified for the transfer bar.
It needed a front grip frame screw and a grip screw. I called Ruger in NH and tried to order these screws, but since the gun had never been back for the transfer bar safety installation they refused to sell me the screws.
I hung up and called back and ordered the same screws for a New Model and they were pleased to send them to me. (The shipping cost more than the screws!) Yes, the New Model screws fit the Old Model just fine!