Hmmm, those seat post sizes, if correct fit, point to something with "heavier walls" than regular 531 "C" and could be Tange or Ishiwata as you surmised. Which makes this being a "custom" more likely, some builder who mixed tubing, used some Asian-sourced frame bits as well as Euro, kind of more the "American" practice than British. Then you've got the metric pinch bolt and 27" wheels (but this is not a "touring" frame from the look, despite one set of eyelets). Strange bird, indeed!