"Concerning the "revolving" adapter: yes, it is a Sinar advantage, definitively. But there are people who do not want it, respectively are not ready to pay that extra money for it (though it is not that much), respectively it makes the "Sinar Hy6 - 65" cheaper."
Not that much is a relative term. I would have bought the revolving back, but I was quoted its price as being 2500 dollars. Since the 54LV I found was 10,000 (used), 25% more to the cost was not a trivial amount, and certainly not worth it just to rotate the back, which you can do rather easily just by detaching it and reattaching it in a matter of seconds. I think Sinar made a good decision here. Let the consumer decide if it is worth it to them. For people who shoot mostly in the studio or mostly landscapes and tend to stay in one orientation, the ability to rotate it is not necessarily worth the high cost. For others who go from landscape to portrait orientation frequently, then they can spend the extra money for it without reservation.