Celeste green gives it away.....
I should have said it's not a Bianchi. It's painted as one but, alas, it's not.
Colnago has come up with some wierd stuff over the ages, including bikes with double down tubes. Wonder if this is an addition ?
This is an old frame...the teeth are used with a Campagnolo "Cambio Corsa" 2 lever (in the rear!)shift system. Probably pre-dates the start-up of Colnago (1953).
The twin lever Campagnolo was used in 1948, Bartali in T de F,but the improved single lever was 1949. The mystery frame has a cheap lug set, legnano? mass produced?I raced in the early 1950's with a single lever Campagnolo Professional, as used by Fausto Coppi in the 1949 Milan - San Remo illustrated in Bianchi Catalogue about 1952. Memory a bit vague.