If I remember correctly from when I had a '62 Specialisima, the first number is where it is in Bianchi's line up with #1 being the top and so-on down the line and the second number would be the second digit of the year i.e. 2 for '62, 3 for '63 and so-on. My Bianchi also had the cool integrated headset like that one but much nicer and chromed. So I'd guess from the '60's or earlier.