John wrote: "All helicomatic cassettes were interchangeable. The 720 used the top of the line model, with ground and polished races and chromed nuts, rather like the Mallaird 700 series, IIRC."
It is not true that all helicomatic cassettes are interchangeable. I too believed that they were all interchangeable until I sold one on ebay. It turns out that there are in fact two different diameters. Apparently there was a later heavy duty hub made for MTB with more sturdy ratchet mechanism.