Rendezvous club speed dating
This clearly is when the component was made and not when the bike was made, but unless the component or bike manufacturer had lots of stock lying around in inventory, the date should be a fairly good indication of the year of the bike.
At least it would be the earliest date that the bike could have been made.
Pull the lever and look inside the top of the lever arm for a code such as "1084." Dia-Compe extension levers (yuck) also tend to have date codes on the side that faces the brake hood.
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.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.
Unfortunately, many of these are coded, and require some additional knowledge to understand the code.
If you know of other components that are marked or coded that can be added to this list, please let me know.: The information on this page is copyrighted.
For old Treks, 27.2 is the most common, but for other bikes the diameters can range from 25mm to 33mm.
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.