Ruger's serial number lookup page will only show serial numbers that were entered into the program digitally. Before that you need to go to the old version of Ruger's "Dates of Manufacture". For example, google "Ruger Bearcat Dates of Manufacture" which will send you to a page that shows a general year by year listing of ship dates. If you want more specific information, you would need to get with the factory to send a factory letter as mentioned above.
As for Ruger's serial number lookup page, the digitally entered numbers go back almost reliably to 1984/1985, and I have found dates as early as 1976 so far. But for some reason some series can't be found at all... for example any of Ruger's blackpowder models.
Chet15