My guesstimate is 1978-1979. This is based upon 1) examining the 1978 catalog, as well as picts of 1976 and earlier Bianchis that have participatied in Eroica, which show the chainstay cable guide on top of the chainstay, and 2) picts of a 1979 and my own 1980 super leggera/super corsa frame that have under the BB cable routing.

OTOH, as BianchiGirll suggests, this may differ depending upon the specific models.

stan