1961 is different than all the later ones. it has a vent tube off the filler neck. they have to have non vented cap or the pressure in corners will blow fuel passed the vented cap. the tube in the filler neck allows the fuel topush hard against the cap because the tube lets atmospheric pressure into the tank. on the later trucks with vented cap there is less pressure behind the rushing fuel in a corner because there is no vent tube allowing in atmospheric pressure. hope that makes sense