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.

You may be confusing the 1971 NOX with the later OSAC. NOT the same thing.