RSC ends their Bianchi city bike registry at 1980. They have over 100 registered and none use the letter "H" in their serial numbers.

That said they have several starting in the 70s with a similar format. For example, 3A04178 is a 1973 Onice, 6C05301 is a 1976 Ghisallo. So the first digit is the year, the letter could be a model, but there is not consistency with the letters to that regard. So your bike could be 1977/1987/1997/etc. Can't say for sure, not enough information. Although it is probably not earlier than 1977.