Marty,
Was the mustang a 68 6-cyl coupe? If so, Jeff and I took 100s of pictures of the car several years ago in Oklahoma. Lots of detail markings from the factory on that mustang.
Based on the "strike" picture gallery, the turn signal grilles sat horizontal on a painting frame with the stone guard so I am less inclined to believe the nut was used to hold the grille when painted. But then again with high pressure painting, maybe they did need to hold the light part to the frame! I agree with you that the flat nuts appear to be painted for this car. Hopefully, Jeff will find some reasonable pictures since he is the king of pictures and San Jose cars.
Regards,
Pete Morgan