The "Standard Catalog of Smith & Wesson" third edition, AKA SCSW3, "The book", etc, lists the dash changes by model. What a particular "dash" indicates depends on the specific model. SCSW3 is available from many on-line and conventional book sellers, just Google the title, including the "", and take your pick of sellers based on price. You should find it in the $35-40 range. If you have any interest in firearms this has become one of the basic reference sources. Buy it, you will not regret it.
yeah buffco you do need to buy that book as everyone has said. lots of knowledge in there. you think you have an interest in smiths now that book will turn you into an addict.