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For example, the back of cranks are usually marked with crank arm length in mm, typically in the range of 165 to 185.
Seatposts are marked with outside diameter, also in mm.
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.
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.
Many components are marked with size descriptors in addition to component manufacturer's date codes.
Especially useful, and challenging to sort out and verify, was the Shimano code.
Also gathered on this page are date codes decoded and generously provided by others.
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.