N900000 1982-1986
Highest known range is N97xxxx (ca. 1986)
* Approx. the last half of the N200000 range and most of the N500000 range consists mostly of M28s & M58s. After 1980, the M28 serial numbers were merged with the other N frame s/n ranges. Thanks to John A. Crawford, Ph'd., for this N prefix update.
Supica, Jim,Nahas, Richard. Standard Catalog of Smith & Wesson (Standard Catalog of Smith and Wesson) (Kindle Locations 16864-16867). F+W Media. Kindle Edition.
1982-1986 according to my "book".