On my 66 "K" engine are two radiator hose marked with the current hose part numbers, C6ZE-8286-A lower and C4ZE-8260-A upper, plus a Falcon service part number, C3DZ-8286-J (?) lower and C3DZ-8260-H upper. This are about the only parts I know of that have an engineering, aka assembly line part number, and service part number, the "Z" in the fourth position, that show up on Mustangs. I got these from Marti Auto and I know that Kevin did a lot of research on the parts he sells, still, did Ford use the dual part numbers as described on these parts?
Jim