The 520 was a CrMo double-butted tubeset, 9/6/9 TT and DT, and 9/6 seat tube - just like Columbus SL! The 620 was Reynolds 531CS main tubes (see the Trek tubespec in the front of the 1983 catalog on the Vintage Trek site), and some kind of CrMo forks and stays. The 720 was fully double-butted Reynolds 531, but a stiffer 10/7/10 downtube, very long chainstays, and different brake specs in different years. All are great bikes!