Supposed Herman Miller 1970's Space Age Prototype  

I saw the ad and almost...
I saw the ad and almost reached out to the seller. I could not however handle the fact that some kook was trying to run an auction through criagslist. If you want to auction something, put it on ebay.
Hope you got a good deal. I looked around and found an auction result for another version of that chair with a lower back. It's seemingly legit Herman Miller, but rarity does not necessarily equate to value of nobody wants it or knows about it. Cool nontheless, congrats.

Well
apparently there was at least enough local interest that several people were willing to pay significantly more than I was, anyway...
It's a cool little known underappreciated rarely seen chair, I don't even see much argument against the "prototype" claim, at the very least it sure wasn't widely distributed at all...
Nice chair! If only no one else noticed/cared and I could have bought it for $67...

Blue Monster II
If you had a chair museum, it would definitely be worth a 67 dollar flyer. Especially if it were a prototype of some kind. It IS a bit of a "beast" though.
A cool beast. I find the base interesting, grotesque and a bit humorous as well. Like the designer that lost his way a bit, or arrived late to the party.
The shape of the chair feels to me like it could lose a few pounds.
I wonder if we will ever see another one.
(edit: Thanks Pegboard, I didn't see your post until after I wrote this post above)

That's a Ray Wilkes design
isn't it?
And it's a production piece, not a prototype. They don't put manufacturers labels on prototypes that are not intended to be sold to the public.
It's a fun chair, but not a hugely important design.

Whatever you wanna call it
It is not unheard of for oddballs and one-of-a-kinds to sometimes make it out into the world (through the mythical HM company store for example) with labels.
I don't know if this is THE example of this particular variation of this chair, but it's the only one I've seen anyplace. There doesn't seem to be a history in the marketplace. The story doesn't seem implausible. I shrug my shoulders.
A sample manufactured for the designer's uses would be prototypical.
Obviously this is a dirty word around here for a reason though.