My 68 GTX has headrest on both sides. Oddly, the 69 Charger SE my brother has doesn't have any, or a 69 Hemi Bee. Both of them were low miles, original cars.

I think head rests were standard somewhere mid-69, optional from there back.

There used to be a 67 GTX close to me that had factory headrests as well. car is long gone now.