You are correct, all 1962/3 F-85’s came with seat belt anchor nuts welded to the floor pan for the front seats. If still original, the nuts will have rubber plugs in the bolt holes. I have not seen any with rear seat belt anchor nuts in the floor pan. They only showed where dimples were put into the rear floor pan to drill holes for rear seat belt installation. The dealers could order seat belt kits for the car and install them for the customer or you could do it yourself. These instruction sheets came with the seat belt kits that you could purchase from the dealer and install yourself.
