Chad Hiddleson is the editor for the Red Eagle News Exchange (RENE). A $25 subscription will get you a two volume reference manual with every known Ruger variation in all models. You also get a quarterly news letter with articles like the SHOT Show, Tulsa Gun Show, etc. I highly recommend RENE, even if you're just curious about when your gun was made. The references include all serial numbers and years of manufacture for all Ruger models.
The hard ones to track are the Davidson's and Lipsey's special order models. They are not cataloged items. Here's where it's nice to know if your gun is a factory original or if someone modified it.
You can contact Chad at CHRugers@aol.com if you are interested in a subscription.