I had an Inter-10 that I believe was mid 70s judging by the components. Like yours, the only 'braze-on' was the integral dérailleur hanger, it was built with Shimano SF dropouts, and it had the same half-chrome treatment. It was full double-butted 531 (main tubes, stays and forks), the seatpost was 27.2, and the BB shell was English thread. The s/n on mine was 5604521.