My book says 1992 for the BKZ prefix s/n.
1992 – BFV, BHP, BHT, BJB, BJC, BJD, BJH, BJJ, BKB, BKZ, BMA, BNA, BNB, BND, BNE, BNN
Supica, Jim; Nahas, Richard. Standard Catalog of Smith & Wesson (Standard Catalog of Smith and Wesson) (p. 491). (Function). Kindle Edition.