Piero,
According to Anthony Curzon (and others), the 5 rib "interim" hub was only used in the AMC twins range in 1963 (and the G80CS to '67 as Knut pointed out) and the final "Ranger 750" version P11. That may attribute to the scarcity of the 5 rib "interim" hub. The earlier AMC (non-"interim") heavyweight hub is wider and has more fins. There were Matchless and AJS models built for many years, so the early AMC hubs would be likely more plentiful, particularly in Europe.
Again, I am not an AMC expert, so if this information is incorrect, I invite comments from those who are.