"Rad" is wheel and since it is associated with the city of Osnabrück where the Karmann factory rexides, a reasonable guess is that it refers to something made with the original Karmann badge which had a wheel on it.
RAD is Reich Labor Service. Reichsarbeitsdienst (RAD)
Seen the old films of guys in uniform marching around with shovels on their shoulders? They were members of RAD.