Wanna check your book's accuracy? My S/W 940 Centennial 9mm 5 shot snub ser # is BFV689x. Left S/W on July 16, 1991. How close is your book?
I assume that you have a letter from Roy Jinks, S&W Historian that gives you your shipping date.
I will also reiterate that the authors state that there are discrepancies in S/N dates in the SCSW 3rd. ed. due to a variety of factors causing gaps in S/N sequence or S/N prefixes being used out of sequence.
The SCSW reports that your Model 940 9mm Centennial Stainless is a:
"Caliber: 9mm Parabellum. Double-action-only revolver built on the round butt stainless J frame with three screws. 5-shot fluted cylinder with a length of 1.51"; 2" or 3" barrel, 1/8" serrated ramp front sight with square notch rear sight. Fully concealed hammer, .312" smooth combat trigger, satin stainless finish, Uncle Mike's Santoprene combat grips. Supplied with full moon clips. Cataloged 1991 - 1998."
It also reports on the Model 940 Special
in .356TSW caliber.
Engineering and Production Changes include:
"940 (1991): Introduction.
1993: 3" discontinued.
940-1 (1993) New firing pin
Since your gun is a 940, the engineering and production changes alone narrow it down to 1991-1993.
The SCSW 3rd. ed. "Serial Number Range By Years" Appendix places your gun's S/N into date areas: between August 1990 and January 1991, or in Sept. 1992.
The SCSW 3rd. ed. is by no means perfect, as there were a lot of guns moving through S&W and no one person in charge of documenting anything. The book was compiled by S&W experts and uses information compiled from many sources. I do think the Standard Catalog of Smith & Wesson, Third edition, by Supica and Nahas, is an invaluable resource for the S&W enthusiast.
Hope this helps,