Thanks! Mystery seems to be solved; they are a Cougar part which seems to somehow to have found its way on to my Mustang. Probably due to assembly line shortage. There are several other oddities on my car probably also due to assembly line shortages; standard interior with a delux (woodgrain) steering wheel, and a type of GT fog light switch that I have never seen on another factory GT.
And, yes, my car was purchased new from a local dealer's lot.
On another subject, does anyone know what the correct radio speaker grilles for a 1968 with AM/FM stereo and standard interior? I have seen pictures of cars with this combination using the deluxe interior full length lower door grilles, but don't know if factory correct.