You are correct that Model 610s never stayed in S&W's catalog for long. My most used match revolver is the 3 7/8" version of your Model 610-2. If I recall correctly, your 6 1/2" Model 610-2 was the first cataloged version. My shorter barreled but other wise identical Model 610-2 was cataloged a couple years later. Both barrel lengths were not cataloged at the same time. Model 610s fell out of the catalog then came back again with fluted cylinders and locks.
Model 610s were slow sellers. I bought mine during "S&W Days" at a local gun store. Factory reps were on hand to help with the promotion. It was $486 plus tax. I was not lucky to find it at that price. That was the most retailers could squeeze out of one. It just supports the theory that today's unwanted turkeys will become tomorrow's coveted collectors' items.
While it will not help you with this revolver, the once secret code under Special Order was the Julian date the revolver was boxed omitting the decade. 1234 would have been the 234th day of 2001. Presumably factory employees would know it was not the 234th day of 1991 by manufacturing changes.
621 is your revolver's assembly number, or part of it. All S&W hand ejectors have an assembly number stamped on their yoke, frame and side plate to keep those fitted parts together during manufacture. To make room for model numbers, about 1957 the frame assembly number was moved from the yoke cut out to the left side of the grip frame.
