Vented -vs- Unvented Gas Cap 72 Scamp w 318

Hoping someone here has an answer for me. My buddy owns a 1972 Plymouth Scamp with a 318. The car has a chrome gas cap. He was at the Mopar Nationals and was wanting to purchase a replacement one and was told by 2 different vendors that they would not work on a 72 ? It currently has a chrome cap on it now? Does this year car/engine size need a vented or unvented cap and could some one please explain the difference to me? Thanks in advance, Ron