Gas Cap

Generally all gas caps are vented on mopars with carburators. Without the vent, as you sucked gas out of the tank the tank would also get sucked in. Think of a paper bag you blow into, if you inhale from the bag after filling it, it collapses. The cap you bought is likely a 74-76. The filler necks changed slightly when the unleaded flapper door was added in late 73/74.