I had the vertical tube type on my plus 4 Dave, a threaded and tapered spigot sits behind the over rider with a nut on the back and the tube from the rack just pushes onto its taper. Occasionally I had to push it home as the tubes work their way off the taper a bit.
It's good to be able to store the screens but down side is the rack sits quite high as its 'thicker'.
I also turned a thick stainless washer to sit between the threaded T bar and the rack, the polished flat plate marks very easily.
Sorry no pics though.
