I may have figured out how custom-built Bianchi road bike serial numbers work (at least in some instances).

There is a long-running debate on the forum about SBX frames with late 80's stickers. SBX is from 93/94 so this causes consternation...

The serial number is the key (I think anyway): the format "B" XXX-XXX is reserved for custom builds. "B" means Bianchi - made in Italy at the Bianchi factory. The rest of the number is simply the frame number increasing incrementally - starting at "000-000" and going to "999-999".

My SBX has late 80's stickers and the serial number is B 008992. Other posters with similar "weird" SBX frames have B 008919 and B 008905. These frames were probably made close in time together but they are each custom - geometry, cable routings etc. These bikes are all SBX - so probably made in 1993/4. Custom frames are notable for design differences from the production bikes (decals, simpler brake mount, no pantographs etc.). It seems the Bianchi design philosophy for "custom racing frames" is to keep it simple - no fancy stuff, just function. A bit like a racing car perhaps?