My 73 charger was untouched at the sending unit and it had a flat seal. I never seen a o ring type. I`m sure as stated the part number on the seal didn`t change until they went with the larger sending units in the later 70`s. Maybe your example was not the OE seal. I too had the same issue with the flimsy aftermarket lock ring. I dumped about 5 gallons of gas on my garage floor. What a mess.