The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.Other manufacturers of bicycle components have date marks on their pieces.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).
The rear derailleur can often be dated to a year or two by referring to the book "The Dancing Chain - History and Development of the Derailleur Bicycle", by Frank Berto, 3rd edition 2009.)Most early Treks (1976 through about 1980) were sold as framesets.The components were added by the local bike shop or by the buyer. Components also could be swapped from an existing ride to the new frameset.[Atom] [Brooks] [Campagnolo] [Dia-Compe and Weinmann] [Huret] [Maillard] [Normandy] [Nitto] [SR (Sakae)] [Sachs-Huret] [Shimano] [Strong] [Sansin] [Sunshine] [Sun Tour (Maeda)] [Sugino] [Tange] [Williams] [Dancing Chain] [Saddles]The date of manufacture of a bicycle's components can often be used to determine the date of manufacture of the bike itself.Some bike parts have a date code cast or stamped into the piece.Dia-Compe cantilever brakes don't appear to have a date code, but supposedly the 981, 983, 986 brakes were first introduced in 1981, 1983, 1986, etc.