You may already know this, but here's the authoritative online guide for vintage treks: http://www.vintage-trek.com/SerialNumbers.htm

I've provided the link to the page on Serial Numbers, and how to locate/interpret them. Great information on the rest of the website as well, including brochures. Your frameset is likely 1981 or earlier (if those decals are original), and the Columbus tubed frames were the top of the line.