Your best bet might be to contact Ruger directly. I have a "three screw" Single Six bought new in the '70's. If you don't already know, the major difference is that the new model Ruger uses a transfer bar system which keeps the hammer away from the firing pin when the hammer is at rest. With the old style, it was apparently possible for the hammer to set off a chambered round unintentionally. For that reason it was recommended to load only five rounds, and keep the hammer down on the empty. My understanding is that no other major parts were changed, but there might be just some normal production changes to later cylinders. Hopefully someone else on the board may know for sure, or as mentioned earlier, contact Ruger directly and ask them. They do have a web site, but I can't remember the address. You can email them from the web site. Hope that helps some.
Regards - AZFred