Markings depend on when the work was done. If mid-70s or later, there are probably no markings on the gun. Look at the underside of the extractor, if it is serial numbered to the revolver, the cylinder is probably original as a new cylinder would require a new extractor.

There could be a date stamp on the grip frame (for example, 2 70 would indicate repairs/replacements done in February 1970). An R-B stamp on the grip frame would indicate the revolver was submitted for repairs by S&W's Customer Service Dept as it would have been shipped there.

Bill