Long chainstays are used on touring bikes to provide a more stable, cushioned ride and to provide more room for panniers (saddle bags).Serious touring bikes in the 80s and 90s were equipped with cantilever brakes (the classic center-pull cantilevers).On the used market, this often means less money for an even better bike.Only the 85 model had the extra-long 47cm chainstays, matching those of the 720. For example, if you want a 1983 frame with 620 geometry and materials, you can look for a 1983 600, 630, or 640, all of which have the same characteristics as the 620.
The TX900 was the early top-of-the-line frameset, dating from 1976 to mid 1978.
Trek made a total of 7929 720 and 728 bikes and frames (number calculated by Mitch Hawker). The 520 touring bike first appeared in 1983 and probably has made more trans-America crossings than any other bike model.
The Model 170, arguably Treks best steel racing frame, was available during the period 1983 to 1985. It still is offered by Trek in steel (although not lugged), which has helped maintain significant model recognition among bicyclists.
Mountain bikers, and carbon and aluminum aficionados do not despair - the brochure extracts on the site contain a list of specs for all of the bikes Trek produced in that year.
Additionally, the table of Trek models by year includes all Trek bikes through 2005.