I have been doing some detailed research on my '68. One of the subjects I've studied is the as-built tires and wheel covers,
To make a long story short for this mystery:
You could not have the deluxe wheel covers (even though they were part of the Custom Trim Option) with the optional WSW 195Rx14 Rayon Radial Ply tires.
Many of the trim combo items were limited in customization because they were preassembled or came from somewhere else in a package. So they just didn't offer the option of a full ala carte. So maybe the wheels came to the final assembly from different places, or with a deluxe trim package, and the wheel covers were all ready accounted for? In other words they would have had a bunch of extra wheels they didn't need so they just stopped that option.
And then sometimes you could totally customize. It is certainly odd. Other times I think they just didn't anticipate that need/want.
It's possible. I had not considered a factory options restriction. I kept thinking a technical reason. (I ruled out aesthetic reasons).
Oddly, my car was ordered with the custom trim option which includes the deluxe wheel covers but they deleted them when the radials were ordered. It got all the other custom stuff, just not the covers. And, the Deluxe discs are the only hubcap that has that the "not on radial tires" restriction.
Oh, as an aside, I also didn't get the glove box bird that is promised with the custom option but that I guessed as being either an error or out of stock part so they ignored it.
