There are a few around that go up to 1969, but after that Longines restarted at 50,000,000 and I don't know of any site that tracks serial numbers from then on.
This will give you a fairly accurate date for pre-1969 models: http://www.vintagewatchresources.com/year_identifier.php After that, you will need to contact Longines directly: https://account.longines.com/en/contact You'll need to set up a registration with Longines. Hope this helps, gatorcpa
Great service from Longines. They answered very quickly. Ten points to their customer service and to you guys