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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.
The most likely components to be original are the stem, handlebars, seatpost, and brakes.For Treks, the SR date markings are especially important.Virtually all of the Treks not equipped with Campagnolo or Shimano Dura-Ace components, started life with some grade of SR seatpost and may also have SR stems, bars or cranks.Even those equipped with Suntour Superbe components usually had SR seatposts.
Many components are marked with size descriptors in addition to component manufacturer's date codes.
The Appendix gives dates of initial manufacture for the vast majority of derailleurs made from 1920 to 1999.