Did you see this on vintage-trek?
Originally Posted by vintage-trek.com
The Japanese Connection - Beginning in 1981 or 1982 (and extending through at least 1986), Trek used a subcontractor in Japan to build some frames and parts of frames. The 300 and some 400 series were a complete (unpainted) frame and fork from Japan, but painted and assembled in Waterloo. The 500, 600, and 800 (MTB) series had the main triangle built in Waterloo, the pre-assembled rear stays were attached, and the whole works painted and assembled in Waterloo. The 700, 900, and 170 series were completely built, painted, and assembled in Waterloo. Some 600 series were all Reynolds 531 frames. Trek had Reynolds seatstays and chain stays shipped from England to Japan, where they were assembled into the rear triangle assemblies and then shipped to Waterloo to be attached to the main frame.
Sounds like the frame came from Japan to me, but bike assembled here