The hubs were wire wheeled clean and then painted in black hammerite. I hope it's only the paint you can see.
Is the paint an issue? The hubs were clean and mating parts flat after I wire wheeled them.
It does rather look as though your back plates may be stood on rust around the bearing boss, which could distort them as the bolts are tightened.
A wire wheel wouldn't clean it off, you need a small chisel and hammer to shift the scale.
Assuming that the hole through the back plate is the correct size, aftermarket and all that...
1985 Oettinger 3.2 Caravelle RHD syncro twin slider. SA Microbus bumpers, duplex winch system, ARC 7X15 period alloys
Thanks Aidan and Silverbullet, you were both right, I hadn't removed all of the old rusted backing plate. I could barely see it, but another go with the wire wheel and chisel/screwdriver and this bit fell off.
The new backing plate is sitting flush now and the wheel does spin a bit better, but not that much better. Maybe I need to do the rear wheel bearings too?