I need to amend my previous post when I said that neither cylinder had the last three digits of the serial no. inscribed on them as the manual prescribed. I looked again at the face of the cylinders and, as John BT indicated, this time I noticed the extremely light, almost undetectable, free-hand scratchings that are the last three numbers of the serial no. This leads me to suspect that the cylinders are indeed fitted to the frame and that maybe it isn't a good idea to randomly install any given cylinder to any given frame. Sending the revolver back to Ruger might be the best course of action to have a cylinder properly fitted.