Using Velo Retro's Colnago timeline has me more convinced that this is a '78 or '79:
It has the items introduced in '78:
- Fork crown for recessed brake bolt with shallow triangles cut into back
- Brake bridge for recessed brake bolt
It does not have the items introduced in '80:
- Fluted seat stay caps engraved COLNAGO and straight seat stays (no longer biconical shape)
- Brake bridge with square boss for recessed brake bolt