i've also wondered about this, all the ones i've seen in junkyards dont have any, i'm guessing it was an option,But you do need it, it focuses the air coming in, without a shroud it blows it all over the place, i saw a video about it i think it was on yourube.
Mine never has one, my father bougth the mav on ´76 and have been our since. The car never overheated ´cause of it (and the region of Venezuela I live in is a hot one).