What "dash" is your gun? A 66 "no-dash" would have been made long before a BSYxxxx serial number.
By the way, there are huge gaps in the 3-letter prefixes as listed in the book. Short of getting a letter from S&W, you just sort of fit your prefix into the general range of prefixes while keeping track of when the version "dash" models were made. Sometimes the date the woods grips were made is stamped inside the grips -- that date is usually within a month or two of when the gun was made (assuming the grips were original). However, if your gun shipped with Uncle Mikes or Hogues -- then it was made in 1994 or after.