Ruppstang,
Thanks for taking the time to take these pictures and for posting them. The picture of the loose wheel cover is definitely the darker shade. But, as you pointed out, the picture of the wheel cover mounted to the wheel, shows the lighter shade. Perhaps they all start out dark and the ones kept out of the sun and elements stay dark and the ones exposed to the elements fade over time.
The mystery continues but....based on your advise, I'll go with the darker ones for the show (unless more definitive info surfaces).
Thanks
Mike
68 coupe, Dearborn plant.