From a performance perspective these manifolds do not measure up to other manifolds like the LD340 from the same time period or newer manifolds like the Performer RPM. I do not believe these old manifolds are rare.
I have the original article from I think Car Craft. Manifold came out in 1971. And in all of Edelbrocks dyno testing it out performs the LD-340 in every respect. And they did a lot of testing! They made the original one out of fiber glass so they could tweak it between each dyno run! For the day it was about as high tec as it gets. I picked one up yesterday at the Nats for $60 and plan on running it on my 65 dart. As I want that vintage drag car look. Only other real option I had was a new Ofenhauser intake but $350 is rather hard to swallow. Only saw two at the swap meet. Not rare, but harder to find than the newer ones.