Ah, the seats in the pic are Curved neck ones, starting on Camaros in Late Nov 68, but there was some overlap due to stock on hand.
http://www.camaros.org/headrest.shtml
I have both curved and straight headrest seats. I have noticed if you try to put curved headrests on straight headrest seats, they do stick up more than normal. But when looking at the slides and plates, I have been unable to notice a difference. Another mystery for future research!