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I believe that's just a common marking done in production.
If the "S" was stamped before or after the serial number, it would likely be a "factory second". Only way to verify that is to spend the $10 for a letter from Ruger.
Yes, they must have had a bunch if them "S" tamps at the factory, they used them to stamp MANY things, suild have been ONLY to "proof" the cylinder after 'testing',,,,,, maybe there was a "set" , an "S" and an"R" ( Sturm/Ruger?) now they been using the "R" stamp to mark the "safety conversion updates" under the cylinder frame......someone took their "extra/spare", stamps way long ago and gave them an "electric pencil", now try and figure out just what other letters they used along with the last 3 digits of the gun serial number on the spare cylinders over these years.....