I believe that the "S" stamp on the sideplate was done for a short time simply to visually distinguish new from old sideplates. The machined cuts on the back side are different, and pre-safety sideplates will not interchange with those made to accommodate the improved drop safety. I don't believe it has anything to do with post-manufacture conversion. I don't know when the "S" sideplate stamping ceased, but not too far into the post-WWII period.