Yes it is a 6-speed freewheel. I even researched it by looking at the original brochures at http://www.vintage-trek.com/TrekBrochures.htm

Oddly enough it shows the 84' 620 and 720 had 6-speed freewheels and for some reason in 85' they came equipped with 5-speed freewheels. Although it seems backwards from everything I can see they went back to 5-speed freewheels in 85'.

Thanks for your inquiry, it lead to me learning something odd about Trek. Also, yes I am aware that if I go to 700's I will need to cold set my frame to 130. I read up on that also.

Have a great one.