FYI, I spoke with Ken at Vans this AM. Ken says all available S602-1 backplates are dished, and pulling the S602-1 and S602B together won't hurt anything in the practical sense.
With the center removed per a CS Hartzell application, mine was still dished 1/8" when measured at the edge of the cutout. I did a trial assembly, fully riveted. The result is a lot of deformation at the rivet locations; obviously something has to give when you pull a dished part into compliance with a flat part. Personal choice; these parts are now patterns.
Don't think the dishing hurts a thing in a fixed pitch application.