If you take a 66 or earlier bucket apart you'll find the upper member of the back frame is round tubing. No holes for head rests. For 67 and later that frame member is square tubing with holes through for headrests. When I put 67 seat skins on 66 frames I found the back frame with round tubing is a little bit too tall. The binding on the seat skin peeks out from under the steel back in 2 little spots.
