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.
As the wear on a vintage bike increases, the greater the likelihood components are not original, either through replacement of worn parts or through component swaps.
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 rear derailleur freewheel/cassette and chainwheels are probably the first to be changed on a bike.
On a vintage bike in excellent condition (that apparently had a lonely existence in a garage) all of the components likely are original.
( Yi-Chin Lee / Houston Chronicle ) less Houston Teans defensive end J. Watt watches the Houston Dash taking on Seattle Reign from a suit during the second half of the game at BBVA Compas Stadium Saturday, May 27, 2017, in Houston. NOTE TO USER: User expressly acknowledges and agrees that, by downloading and/or using this photograph, user is consenting to the terms and conditions of the Getty Images License Agreement. Watt of the Houston Texans and girlfriend Kealia Ohai courtside during Game One of the first round of the Western Conference 2017 NBA Playoffs at Toyota Center on April 16, 2017 in ... more On an October 2016 episode of the 2 Up Front Podcast, Houston Dash soccer player Kealia Ohai admitted that she is dating Houston Texans star J. "Nowadays, if someone struggles, I just skip the awkwardness and say, ‘just call me K.’”Ohai’s sister, Megan (right), is married to Texans linebacker Brian Cushing (left).