I believe there were only two styles of plates; one for Dzus fasteners (what I guess Geo is calling the removable windscreen) and one for threaded fasteners. The cutouts in the body changed too, so this isn't an area where it's easy to mix-n-match (though perhaps the early stanchions could be used with the threaded fasteners and plates, as the only difference in the stanchion appears to be a smaller hole).
Don't know about VW, but there is a commonly available switch that will fit; ISTR mine fit old Buicks and Fords. Given how well it lasted, though, a working original may actually be a better bet.
However, it appears that Herman still needs the unions.