I believe the difference between the QB2 and QB3 is the same as the difference between Stingray Mk1 and Stingray Mk2...

Mast upgraded to Wingmast (mostly Superwing.. same section used by F16 today!), and extension by 2-3ft, Increase in sail area from ~200sq/ft to 225sq/ft

I suspect these boats had no minimum rigged weight, just platform weight, the stingray was 88.5kg... I think the QB was lighter!