Looking through my pictures I'm simply amazed by the number of 68 Mustangs that didn't get power brakes
John here are a few examples of what I found - not the one I recalled but the survey speaks for itself I think
Found two with the white (painted black) style valve but both pretty early cars - not near your cars' date
And all the rest were the black with the long center "stud" including our Cal Spec which is fairly close to yours (approx 5000 numbers off)
Have more with this style - figured this was enough
Hope this helps