Of the 1010 Boeing 707s that were built between 1957 and 1991, 154 were actually designated "Boeing 720" - a medium range, shorter version of the plane. Nowadays, however, we often treat it as merely a subtype from what I see. I think we can agree (note my thread of 2 days ago) that the 707's "prototype" (the "Dash-80" or the 367-80, not included in the 1010 planes) and the similar looking KC-135 tanker were truly different models and not 707s, but the 720's differences don't really seem enough to fully qualify it as a different model. Originally it was supposed to be the 707-000 anyway. For some reason, they changed the designation.
So my question is twofold. First, why did they call it the 720 and secondly, are the differences really so different that it deserves to be given a separate type number? For example, is it more different from the original 707-100 than the 707-300 is?