I did buy this with the intention of of building building fixed gear. All the components will be transplanted from an old Fuji frame I wrecked in a crash (but miraculously all the components survived unscathed, including the wheels).

I agree with your guess of 70's. I'm thinking Japanese as well due to the Shimano dropouts. Was it common to see columbus tubing on Japanese frames? That is the only thing that seemed out of place for me.