So I got my missing groundcontrol washers, and ended up with a set of ones meant for the spherical conversion (bought them before changing my mind). it's an interesting comparison
the GC ones are 1.5mm thicker, and have an odd lip on the flat side. i was going to say that it mates up with the spherical bushing inside the arm, but it doesn't -- the opening is smaller than the OD of the bushing. hopefully they aren't just wrong, but not sure what the purpose of that cut out is.