Thanks for all the info. I waded thru many of my old photos and finally found a few examples, 3 San Jose cars, Dec '67 thru April '68.
No ones of the current Dearborn project (misplaced them or just failed to note this detail), and there was no clear pattern.
As the clearest photos were as shown above, all on the back side of the rear shields, I just did that. As it appears a bit of a crap shoot.
The fronts seem consistent with the rubber on the side facing the wheel.