Well here is my theory. The factory used the grey on the horn grills and the turn signals so as not to clash with the silver trim (blend in) from 65-72. When they went to the black trim on the signals and the horn grills it makes sense to me that they would have then switched to black seals to again blend in and not clash.
That's my theory and I'm stickin to it. :D
Oh and I bought these with the understanding that they were a very early take off pair because the owner wanted to "update" the car to the 73 look.