My car has them on both sides. Like others have found, there are no markings whatsoever on them. It's a March 65' built Dearborn V8 convertible with factory PS, AC, and a C4. The car would have been on the heavy side with much of the weight bias being on the front. The only reason (theory) that I could think of for using them was to help offset the weight on the front of the car.
Perhaps this info might someday help to solve the mystery of their original usage.
The answer lies in two places. First: The 1964-65 Mustang Chassis Assembly Manual (AM0005 pgs 7,8 and 70) where it is identified as Ford base number 3A365 on pages 7 and 8, and Ford assembly line part number C4DA-3A365-A on page 70 as a "shim".
Second: The 1968 edition of "Ford Car Parts" (aka MPC), Illustration section 30, page 2, magically and mysteriously changes the assembly line suspension part number to a service stock number in the spring classification, C4DZ-5355-A. If you look up C4DZ-5355-A in the text section it says look in the spring charts for application.
Still with me?
The uses with springs are mainly for convertibles with certain "options": A/C and P/S and smog (66); convertibles with the 289 HP and smog (66); some HP engines.
I made a quick look for Falcon use but there wasn't any.
Books are your friends. Please learn to use them.
Jim