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It usually is easy to identify a bike frame as a Trek, even if it has been repainted, as virtually all of them have "TREK" conveniently stamped or cast into one or more places on the frame.
Two parameters are specially important to the 520 over the years: chainstay length (as pointed out by Robert Cooke) and the type of brakes used.
The unique and distinctive design, and relative rarity, adds to the value of these frames.
The 720/728 touring bikes, dating from 82-85 were (and still are) highly-rated bikes for serious touring.
The primary purpose of this site is to gather and disseminate information about early lugged, steel-framed road bikes made by the Trek Bicycle Corporation, Waterloo, Wisconsin.
In response to popular demand, the site has expanded to include not-so-vintage Trek bikes of all kinds as new as 2012.