Purchase a book: " Standard Catalog of Smith & Wesson (Second Edition) by Jim Supica & Richard Nahas. "
It has all the information you want on your guns, and almost every S&W made since 1857. For example: The model 629-4 was made between 1993 through 1997. Changes over time included, changes to the butt (going from square to round); modified rear rear sight, extractor and thumb piece; Hogue grips (on some guns); providing a blue plastic shipping case on round butt models; and providing an external trigger lock.
Since the book has provided clues to scarce (and sometimes more valuable) variants it has more then paid for itself.