There have been reports and posts about S&W revolvers not normally seen with lanyard loops that have had them fit by the factory. In those cases the serial number was re-stamped on the side of the frame, left side, as you say yours is. The conjecture being that the gun was returned to S&W to have the loop retro-fit. The other supposition was that a gun was ordered with this feature, a completed revolver of the correct conformation was pulled from stock and the lanyard loop fit and the gun shipped. In the first case there should be a service date stamp found on the left side of the front grip strap, something like 6.65 for June 1965. Possibly for the second scenario too.
It would be interesting to know of the hole the lanyard loop is installed in is bright, or if it was anodized indicating the frame was finished/re-finished after the lanyard loop was installed. I would think if it was finished after this is a virtual guarantee the work was done by S&W. I wouldn't worry a bit if the remaining portion of the SN matches what is stamped on the side, especially if the stamp font matches the original number.