Old Model Single Six
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!
For your information, those parts are available from Brownells without the hassle.
Also, many parts for the Old Army can be used, and these are available from Brownells, also from Numrich (Gun Parts Corporation).
If you so desire, the Old Army grip frame can be fitted to your gun, and its all steel, but will require fitting and blueing. And steel ejector rod housings are offered, as well.
As to grips, CLC custom grips are superb. You might try http://www.clccustomgrips.com/