Code for dating
Swaps can take place early in the life of a bike as the proud new owner upgrades to new or used components of higher quality.
The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.
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.
This makes dating the components an interesting archeological investigation, but one not necessarily related to the date of the bike. Trek owner Larry Osborn made this observation, and suggested this as a supplementary way of dating a Trek (and other bikes as well).
I have a set of Dia-Compe mountain levers where if you pull the lever all the way, a piece of the lever is exposed, which has a clock-type date code.
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.(Don't all cyclists have boxes and boxes of old components in their garage?