No way to know. The factory may have used up the straight headrests before they went to the bent and on the lesser used interior colors, it may have taken longer to do that. Officially, they started using the angled headrests in late November of 1968. However, when they made a change like this, the remaining stock would have normally been moved to "service" status, but there have been original cars with lesser used interior colors observed built later than November with straight headrests.
Ed