That's the Reynolds 531 Butted Main Tubes decal. From what I understand, that is the Trek factory serial number format for the 61x series, though the year code doesn't follow the normal system, which complicates exact dating. Given the presence of top tube brake cable clips, I'm assuming it is 1981, as this catalogues doen't show the models, but the 1982 catalogue shows brazed-on top tube cable tunnels. Both the 613 and 614 appear to be identically spec'd for the crankset, though neither are triples, as on your bicycle. Hopefully, some of the resident Trek experts can provide further information or correct me if I'm wrong. Treks really aren't my forté.
Edit: Oops, looks like I was composing, while Tim was posting. Sorry for the duplication. At least we're on the same track.
