Hello Chris,
Good question, dunno the answer, it was a long time ago! I'm guessing that, as in the case of the Chris Foss Phase 6, the difference between the two versions was that one had a fully symmetrical section while the other was a bit more forgiving (other than when inverted anyway) being less speedy than it's slimmer brother as the section tended towards being a bit thicker and flat bottomed (unlike most people after a Christmas and New Year of overindulgence in the dining room. I speak from experience being also a little more bow fronted than usual).
Happy days!