sloar In the footnotes from the vintage trek site:

"For M and N leading serial numbers, if the TREK letters on the seattube have no colorwrap behind, the bike is from 1980 or perhaps 1981. If it has a colorwrap, the bike is probably from 1982." This supports T-Mar 's observation that there are no braze ons for the brake cable on the top tube. My 1983 600 has those cable braze ons and I would have expected them on an 83 61x.