I think 171 is 1971. In RSC's Bianchi registry, the Bianchis from 1971 are stamped 171 XXX. 1973 is 173 XXX. 1974 is 174 XXX. But there is a single example of a 1974 bike with only a 174 stamp, no additional 3 digits.

All of the bikes from 1975 have an entirely different serial number convention, as do the bikes from 1969 and prior.

No pictures of 1971 bikes in the registry. The 1973 bikes have a blue decal and "modern" font. The 1970s bikes have the "old" font but on a foil sticker, red outline, white fill.