I have just bought a bare frame Bianchi (once in celeste) with a six figure serial number similar to yours in London. The only thing identifying it as a bianchi is the name stamped on the rear dropouts. The interesting thing is that the serial number is stamped on the stearer tube. I guess it is 1984-1985 because it has an Italian bottom bracket. Although it looks similar to several Bianchi's in Fixed Gear Gallery it's not identical to any of them. It has the top tube braze-ons and one for the rear gear cable otherwise nothing else. So it was once racer not tourer, but, its not that light and thus I dont believe it to be special in any way, but it is quite well made.
Looking forward to making it into a street sharp fixie.
Is your number on the stearer too? I haven't seen anybody mention this placement elsewhere.