There's lots of other differences on the Cenntenario too, like a unique serial number format, bottom bracket cut-outs, a brazed-on front derailleur tab, chrome, etc. However, I didn't want to confuse matters with a lot of detail.

The 27.2mm post suggests a Matrix or SL based frame though it was spec'd as Bianchi Speciale tubing during this period. It's really unfortunate that the fork is gone as we couild have used it determine if the frame was an upper model with a full SL tubeset or a mid-range model with tretubi SL or Matrix.

Currently, I'm leaning towards a mid-range frame on the basis on the eyelets and the fact that the other upper end models like the Mondiale and Campione appear to have used Campagnolo dropouts. Forum member Bianchigirll may have some additional insight.