I go into a buddies shop and see a nice Duster. Or so I thought

Rainy, I have to disagree with you on this one. My 71 Scamp had it mounted in the same location from the factory. Not to mention I have seen other 71's with the valve as well. Now unless your talking just strictly Dusters.
Not sure, I just sold a 72 340 Duster that did not have it there?