The PT (Pro Touring) series was a series of touring bicycles from the early 1980s. I have some info on the 1983 PT5000, which appears to be the top of the line model. The PT5000 used the Tange #2, butted, CrMo tubeset. I supect the PT3500 is more mid-range. It may have used the same tubeset with lower grade components, but I'm not sure.