These old vent cranks are a problem. They tend to gall and are difficult to maintain. I would highly recommend that you take the big leap and replace the vents with something like a FantasticFan. It takes some time, but you can see here that it's a doable do. I think you'll find that three vents are not necessary once you get a fan installed that can really move the air. As a matter of fact, I think one FantasitcFan at one end of an Airstream is adequate to move enough air to ventillate the whole volume--window(s) open at the other end.
If you want to stay original, check back with me in a few weeks. I just picked up a '75 Safari and will be replacing the vents. I haven't had time to take a look at them yet, but they may be compatible. Right now I've got to get the Sovereign
and Overlander back together so I can move them. The Safari is a little further down the project list...