May I put forward an alternative theory? Given this frame has a number hanger, is it possible this was built with no braze on derailleur hanger? I know that at times certain team frames (TM) in this era ran a clamp on front derailleur. My theory is that the front derailleur mount was added later when it got it's green colours. Are there any markings on the opposite side of the bottom bracket shell, size, letter H maybe?