gas tank filler neck issues

Sounds like a previous owner added the extra two screw holes. On my 73 sport it uses three phillips head sheet metal screws that thread into the quarter panel recess. There is a thin white closed cell foam gasket that goes between the filler tube and the fender recess. The tube itself is inserted thru the quarter from outside, through the floor gasket, and then into the gas tank. I would be very surprised if yours in different originally than mine. Very little changed year to year on these cars.