Cees, I know what you mean, but things seem to be rather complex. I found Martin Hepperle's [link=http://www.mh-aerotools.de/airfoils/winglets.htm]winglets explanation[/link] very helpful.

Yes, the end plates are not in the slipstream and the vertical tail is. But the thick vertical might be intended to remedy a mushy rudder around neutral. And the rudder has to be in the slipstream to have rudder effect also without airspeed, only by exploiting the propwash. By the way, I think a more articulate rudder could be achieved as well by simply not sharpening it (a slab rudder with square trailing edge).