I think the "S" was to denote "hammer block safety" and also that the sideplate was the modified one with a groove cut into the metal. This groove lined up with the hammer block safety.

Maybe the revolvers returned for re-fitting had the "s" stamped on them to denote the safety modification. I wonder if a new side plate was installed or the old one re-cut...??

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Linda
SWCA #1965, SWHF #245