Probably mid seventies.
Didn't they just go from the spring type cushion/backrests and straight to the Bostrom Viking T Bar style on most models? Mid 70's like Rome said? Or were there some that still used a bench style seat? A member here has several NOS Caterpillar seat cushions with the springs that he had purchased at a liquidation sale. I bet those "Real Deal" items are worth some change for someone wanting an original seat.JM
If the member with the seats has any for a D8 13A with the larger gas tank and so only a single seat, I would be interested in a price for the seat and seat back. Thanks. Michael