I found this Lancia memo, but it is confusing.
Its outlining some aspect of how numbers work with letters. Seems incomplete to me.... but perhaps someone can figure out what this is saying.
This doesn't answer the questions Chris is asking - why no B11, why B15, and why are the B21 and B22 grouped with B20 and not B10?
It seems they were trying to figure out a system, and got caught halfway. There may be a reason to avoid B11 (possible confusion with BII?), and B21 and B22 seemed to have the "hot motors" and so got lumped with the B20...
Its certainly not clear, but then again, we only have a small part of the story. The good news is that Lancia gave us a way to separate the true afficionado from the casual: only the committed can remember the model numbers!