The dashes refer to engineering changes made over the years. Every time they made a change they would give it a different dash number.
If you really want to know all things S&W you should get a copy of the Standard Catalog of Smith and Wesson by Jim Supica and Richard Nahas. All this info is in there as well as serial number info, which will give you a year the gun was born (You didn't really think we had all that stuff memorized, did you).
This is a great read and you'll really enjoy it.
Jim