Hey, Smitty, I've got some interesting news for you.
The Old Model Ruger centerfire revolvers had "counterbored" cylinders.
That means the back of the cylinder is "very close" to the "breech face" of the cylinder frame.
Non-counterbored cylinders have a large gap between the back of the cylinder and the breech face of the cylinder frame since the "rim" thickness of the cartridge is NOT enclosed within the cylinder. Those cylinders will allow the insertion of the "plastic ring" between the cylinder and the breech.
Bottom Line? Your gun has a New Model cylinder. I bet it's been back to the Factory for some work.
JMHO,
flatgate
PS I HAVE several similar examples in my "shooter" box and they are "most favored".