They could have saved a lot of hassle and just done the trucks like they did the early 50s cars. The vent is built in to the filler pipe rather than the cap. It's difficult to see in the picture but there's a stamped divot in the rim at the 12 o'clock position.
Save a life, adopt a senior shelter pet. The three main causes of blindness: Cataracts, Politics, Religion. Name your dog Naked so you can walk Naked in the park.
