81 and smaller. Apparently 91 seems to be unchanged and, of course, IQFoil, too.
Slimmer underbodies in the tail without cutouts for the pure windfoil boards while the X wings retain cutouts (probably for wings to be able to sink the tail for pumping the foil?)
https://www.windsurfing33.com/forum/viewtopic.php?p=913596#p913596
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