Campagnolo shorty dropouts without Portacatena holes, in conjuction with the cables routed over the bottom bracket shell, would point towards 1975-1978. However, it could also be a slightly later , mid-range, Columbus Aelle frame, so check for a 27.2 mm seat post size and the presence of five spiral ridges inside the bottom of the fork's steering tube. If you have these features, then it should be a 1975-1978 Columbus SL model.