As others have mentioned, the -1 (or change numbers, as they're commonly called) signifies an engineering or production change to the gun.
The original 640 is the .38 Centennial Stainless, and was introduced in 1990.
The -1 variant was added in 1995 when the gun was reengineered (frame dimensions were changed) to make the gun suitable for use with .357 Mag. ammo.
The best book for finding this information (really the ONLY book) is "The Standard Catalog of Smith & Wesson" by Jim Supica and Rick Nahaus.
It's a WEALTH of information on S&W handguns as well as other products the company has offered over the years.
It's currently out of print, but the second edition, according to Rick Nahaus, should be coming out around July.