When S&W went with the 3 letter serial number prefix and 4 digit number things became very hard to determine because all alpha numbers series weren't completed in sequence or at all. Your revolver was made in 1990 or early 1991. There is no online reference of S&W serial numbers and the Standard Catalog of Smith & Wesson 3rd edition is the best and only reference book. That or a factory letter for $50 is the only answer.