I almost hate to say but, Ford never made their gas caps, radiator caps, oil fill caps, etc. They bought them like all the other car companies.
If one particular style or manufacturer wasn't available, they used another.
Some fuel tanks are vented (like our Galaxie) so the cap is non-vented. Squarebird tanks are NON vented, so the cap had to be. If you put a non-vented cap on your SB, your car may act like it ran out of gas, especially in cold weather.