The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.
Of course, all this assumes the bike has the original component.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.
Swaps also can be made as the bike falls out of favor, or is being sold, where the higher quality components are traded for lower quality ones that the owner had onhand.
(Don't all cyclists have boxes and boxes of old components in their garage?
The two sets of Dia-Compe brake lever bodies I checked had the four-number date code stamped inside the lever body (I couldn't find any markings on the levers themselves).