That's interesting, I've never seen any backing on them. It may have stopped somewhere between 62 and 69. I have purchased three used doors and had two originals and none of them had any backing on the strips. I don't know exactly what year two of the doors were but all had the push button handles. One of the doors was off of a 73 D500 which still used the sweptline style cab. I can definitely see why a backing strip would be beneficial.