Gas cap confusion

I have noticed after hunting an oem cap for several Fords ( yes whip my *** with wet noodle) that best way to find one is measure the inside and outside diameter of filler neck.\\\

If the tank is not vented ( like said) it will have a tube from it or in case of later B and E bodies, 4 hoses coming out of the darn tank)you need a vented cap and if vented just go with a non vented cap. But like 70 up A bodies each year can have a somwhat different cap.