The best way to date S&W handguns is to get a copy of the "Standard Catalog of Smith and Wesson". It's got the background on the models and variations back to the beginning.
Bullseye competition is fairly specialized. Some of your firearms may have been modified or adjusted to be very light to release. If you have any quesitons, or any of them exhibit "push-off" be sure to see a qualified gunsmith and have them looked over.