Aluminum group with attached ashtrays  

Yes. Those are obviously...
Yes. Those are obviously prototypes. You can tell they were made in the Eames studio and are very, very rare. They belong in a museum.
They must smell of smoke though.

After spending hours scouring the web...
...these remain the only example i could find so yes, i agree they could be prototypes. What makes you think they are from the Eames studio? And not to fan the flames but they don't smell of smoke ; )
Anyone else ever come across unique 'prototype' features on an Aluminum Group series chair?

Why would you think prototype?
Could the ashtrays have been an HM accessory?
It's hard to tell from the photos if it was cast or assembled from scratch. The chromed steel ash box and hinged cover looks like any generic automobile ashtray insert from the period.

Herman Miller design updates to Eames pieces? Fishy I think...
I thought that too however stew on this - the chair is from the '70's while the 901 Washington Blvd office was in full swing. Based on the collective knowledge of the Design Addict forum do we think HM was off doing their own updates to Eames designed pieces? Without (at the very least) design review and approval from Charles and team?
Another puzzlement: Why is the ashtray such a simple design? It looks like it came from a Ford sedan or similar. Although the connection to the chair itself is nicely done and uses same screw heads as rest of chair. Would Eames have viewed this as a functional addition and approved such a straightforward, industrially designed piece?

That was quick!
Good questions.
See my edit.

Hell..Charles did smoke a pipe..and Ray smoked Charles.
just a thought.

Okay, Mark.
That made me want a cigarette.
... after I disinfect my mind's eyes.

Ew...! I'm thinking customized for airports, public spaces maybe?
In the 'public smoking' era I can easily imagine these right at home in a place like Gander airport:
http://viking.twolaneroads.com/the-gander-airport-blues/

pro·to·type ...
pro·to·type   [proh-tuh-tahyp] Show IPA noun, verb pro·to·typed, pro·to·typ·ing.
noun
1.
the original or model on which something is based or formed.
2.
someone or something that serves to illustrate the typical qualities of a class; model; exemplar: She is the prototype of a student activist.
3.
something analogous to another thing of a later period: a Renaissance prototype of our modern public housing.
A clamp-on ashtray attached to a 70s chair does not change it into a "prototype". What you have there is an ordinary aluminum group chair with a smelly accessory on the side.

So Krenit = Snark?
Here I thought your original diagnosis was genuine. So not a prototype, and not museum worthy? Krenit remind me to always reach out to you first for input ; )

For what its worth, the 675...
For what its worth, the 675 Lobby Lounge Chairs at the Ford Foundation Building, "have little ashtrays tucked discreetly under the armrests."
http://eamesdesigns.com/eames-spotting-article/kevin-roche-specifies-eam...

Cool, hidden armrest ashtray!
Cool thanks for sharing!

Thornejc, did you ever find anything out about these? Found a pair today and yours were the only others I've seen