S&W dating is kept intentionally vague, in order to sustain the S&W Collector's Association with proceeds from "Factory Letter" requests. You can get a fair estimate/rough date from "Standard Catalog of Smith & Wesson", but be aware that the 3-letter/4-numeral sequencing skips around among frame sizes, models and calibers now.
The timeline of the change to the present sequencing was c. 1980, at roughly the same time pinned barrels went out of vogue. As in all things S&W, there are exceptions to any "rule" within manufacturing and nomenclature discipline. That said, SCS&W has some guidance, starting in "Appendix B" on pp. 485. On pp. 490 appears the sub-heading--"The Dreaded Triple-Alpha Serial Number Ranges". Res Ipsa Loquitor. They are listed in year-sequential order, and "A dog's breakfast" is a true definition of the scrambled info. Post the 3-alpha sequence and I'll take a stab at it. Text states, and I quote--"Between 1980 and 1983 all models were transitioned to a serial number system that consists of three letters followed by four numerical digits." (pp. 490)