Other forum members are probably getting bored with this since I've posted it so many times. But since the question keeps getting asked, I'll do it again.
Ruger has used only three basic profile plow handle grips since they started in business. You can see them in the drawings below. Your Single Six has the XR3-RED grip profile on it. This is the same as the grips for the Blackhawk, Vaquero and Old Army.
The XR3 was the grip frame Ruger started out with. They brought this frame back again in 2005 with the New Vaquero. There are two differences between the new steel XR3 grip frame and the old aluminum one. The old one was .500" in width and the New Steel one is .437" in width. The grip locator pin is in a slightly different place than on the old XR3 so the grips from the new and old will not interchange.
The Super Blackhawk frame is the large one. This frame has the square backed trigger guard on it. The Hunter grip frame and the Super Blackhawk grip frame take the same grips. The only difference is the the Hunter grip frame has the rounded back trigger guard.
Ruger's Single Six used the old XR3 grip frame from 1953 until it was replaced in 1962 with the XR3-RED profile grip frame which is what your Single Six now has on it. Placing one of your grips in a three inch square will tell you which one it is as in the drawings below.