Question for all spotters: Eames molded armchair, original / fake / valuation  

More shots
photos attached

Ben,
You wont find a valuation here. Sorry friend. Ebay will be your best guide.

thanks, valuation is less...
thanks, valuation is less important than authentication!

Looks legit to me. .
. . . because of the embossed Herman Miller logo.
You can find a lot of information on the Eames office website. Look at the old eames chairs photographed there ? it can help you date your chair.
http://eamesdesigns.com/library-entry/molded-plastic-chairs/
Also, here's a helpful article about dating the rocking chairs, and the meaning of the little logos above the embossed herman miller logo:
http://midcenturymagazine.com/furniture/the-eames-rocking-chair-a-buyers...
The most valuable of these shells are the zenith plastics ones ? they have larger shock mounts and a rope edge. So, yours is not a first generation, because the mounts are too small.

Shell is authentic, base is...
Shell is authentic, base might be a reproduction (thicker main wires, brand new screws), and shock mounts may or may not be original.

Shell is authentic
As noted. Cincinnati Milacron production.

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Indentation on shock mount suggests that the shell began its life atop one of these-

I live in the UK too and ...
I live in the UK too and have 2 chairs with identical markings indicating a Cincinatti Milacron shell as previously suggested; mine are date stamped 1976. The difference is that mine are upholstered in Alexander Girard fabric and have an office style base as pictured in the post above, but with castors. I purchased them for dirt cheap at a second hand store around the time when four footed and four wheeled swivel chairs were banned from the UK workplace (late 90's or early 2000's, I can't remember exactly).
My suspicion is that a dealer got hold of some similar chairs at the same time and removed the upholstery and changed the base to increase the perceived value. I say perceived because it is maybe only the sales price that was increased while the true value was decreased. Nice chairs but they have been messed with so not totally original.

Your arm shell
Looks like the real deal. From the shock mounts and embossing, you're looking at a mid to late 70's arm shell. White arm shells are a rare find. My only concern is that of the first image you took. The shell looked shiny?