The factory manufactured frames in "runs". 100 or 1,000 or even perhaps 5,000. As these runs were done, the frames were each given a serial number. The parts were also given an assembly number to keep all of the matching and hand fitted parts together during the assembly process. Even stocks that were fitted to the frame received a serial number. Once the guns were completed, they were boxed and stored in the vault awaiting shipment. IIRC, the factory even jumped around within given serial number blocks just to keep the competition from figuring out how many guns they were producing at any given point in time.
When an order was received for a given model, the shipping clerk went into the vault and pulled the necessary guns for shipment. The clerk did not pay attention to serial number order as these were pieces of inventory and he only needed a correct number of the correct model to fill the order. The serial numbers were recorded as they left the factory for inventory control purposes but not as a method of dating the guns manufacture.
S&W was in the business to make and sell guns in the present and did not think about how crazy collectors would get at some future time about serial numbers and their relationship to dates.
The books can give you a range of serial numbers used for a particular model and the basic years that these guns were manufactured, however, looking at a serial number chart does not give you the actual manufacture or shipping date. Only the company historian can give you that by looking up the serial number in the shipping ledgers.
We have seen many cases where a lower serial number gun has a shipping date far later than a gun with a higher serial number.
It is possible that somewhere there are records, like the day books, that show what serial numbered guns were built on any given day and hence supply the date of manufacture, but the historian uses the shipping records and thus those are the shipping date.
I hope that this helps. It sounds confusing, but it really isn't if you think about it.
The Smith & Wesson Historical Foundation and its effort to digitize the company records may afford future collectors the ability to search a given serial number and find all relevant company records and correspondence pertaining to that gun. Unfortunately, that is thousands of investment dollars away.
So support the SWHF by sending what you can today!!!