Japan to be exact.....I think that was only the Ishiwata tubing Treks up to the end of 1982....Which are much harder to find than the Reynolds 531 Treks from the same era.
It was rumored that Kuwahara or Ishiwata did the work, which was top-notch. Contrary to popular myth, all treks were hand brazed on certain frame areas until the end of 1982 because the robotic technology did not yet exist to do the entire frame. From 1983 on, the quality declined on some models but any Trek up through the mid-80's is a keeper in my book..