Here's a pic of a 1981 Trek 613; the bike came with a 531 main triangle:
Vintage Trek Gallery - 1981 Trek 613 Steel Road Bike
It looks like daily commute could be right on his ID and this could be a 1981 600 series Trek with a 531 main frame/hi tensile steel fork and stays; the full 531 frames were generally the 700 series.
Here is the brochure:
http://www.vintage-trek.com/images/trek/Trek81.pdf