Please help info needed 67 dart gas cap!

The later tanks weren't vented at the cap, they had another vent off of the tank, so I don't think there's really anything to look for at the filler neck. The later tanks had different set ups, but involved additional lines run either off the front or side of the tank that vented it, some to a charcoal canister in the front of the engine bay.

Not sure this will help either, but at least it shows the type of cap you're looking for...