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Swaps also can be made as the bike falls out of favor, or is being sold, where the higher quality components are traded for lower quality ones that the owner had onhand.
(Don't all cyclists have boxes and boxes of old components in their garage?
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.
Also gathered on this page are date codes decoded and generously provided by others.Weinmann and Dia-Compe calipers usually have codes on the back of a caliper arm.This can be simple like "0784" (month 07, 1984) or a clock-type, with year in the middle and a ring of numbers with a tickmark pointing to the month.""Dia-Compe road levers usually have a code stamped _inside_ the lever.The two sets of Dia-Compe brake lever bodies I checked had the four-number date code stamped inside the lever body (I couldn't find any markings on the levers themselves).However, a Gran Compe set of calipers had no markings. For example, 1182 means the 11th week of the year 1982.