For old Treks, 27.2 is the most common, but for other bikes the diameters can range from 25mm to 33mm.The rear derailleur can often be dated to a year or two by referring to the book "The Dancing Chain - History and Development of the Derailleur Bicycle", by Frank Berto, 3rd edition 2009.Pull the lever and look inside the top of the lever arm for a code such as "1084." Dia-Compe extension levers (yuck) also tend to have date codes on the side that faces the brake hood.I have a set of Dia-Compe mountain levers where if you pull the lever all the way, a piece of the lever is exposed, which has a clock-type date code.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).
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.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.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.)Most early Treks (1976 through about 1980) were sold as framesets.