Could be Japanese: there were a few made with Campy ends and the 26.8 would be fairly common for some Tange tubes (but also for Columbus Aelle if this was an Italian job). It looks like work of a small-output maker since no serial number and these oddball details.
One tiny clue that might clinch it is the HS crown race diameter: 27.0 would confirm JIS, 26.4 would throw this back into "confusion".