Open code dating
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.
However, a Gran Compe set of calipers had no markings. For example, 1182 means the 11th week of the year 1982.
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.
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?