Where were herman miller eames side shells manufactured?  

At a variety
at a variety of different locations. Herman Miller contracted out the manufacture of the shells to many companies over their lifespan. I don't know them all.
Zenith Plastics - earliest - identifiable with a checkerboard logo.
Cincinatti Milicron - ID is a crescent C with a star.
Summits Plastic in Kalamazoo, MI - ID is a pair of triangles overlapping each other (NOT the Herman Miller logo) S or an S with a circle around it.
Vitra, Germany/Switzerland has current production. They are polypropelene and stamped Vitra.
There is also another logo I've seen, but been unable to ID - a pair of flames embossed on the bottom.

LuciferSum - do you know...??
Do you know when Vitra started manufacturing the fiberglass chairs with license from Herman Miller and when they actually bought european rights for the complete Herman Miller / Eames collection??

58 and 2001
In 58 Vitra contracted with Herman Miller and the Eames to produce Eames designs in Europe. In 1993 both companies discontinued the fiberglass production, and it wasn't until 2003 that the polypropylene versions came out.

Here's one for you, Lucifer...
Never come across this stamp before. It quite legibly reads 'Zenith' at the top of the circle with a '2' in the middle. The bottom letters are tougher to make out. But the first letter is a 'P', so I'm guessing it once read 'Plastics'... Thoughts?

On second thought...
Looking at the embossment, the bottom wording beginning with 'P' doesn't look like 'Plastics' at all. I'm pretty sure I'm making out another '2'... Looks like something to the effect of 'PP v 2...'?... Maybe something relegating the chair to a batch possibly?

Oh and I guess I would assume the obvious...
But this chair carries the small shockmount variety. So a 2nd gen Zenith wouldn't seem to be the reasonable assumption...

Lunchbox
Haven't seen that stamp before. Is it an armshell or a side shell?
Perhaps this is a transitional shell.

By the way,
my light blue Vitra polypropylene side chair with the Eifel base sure is rock solidly made and quite sturdy. They might not be original fiberglass, but they did figure out how to make 'em solid.

That's what I was wondering, Lucifer...
I should've taken a snapshot of the entire chair. I was so anxious to get the stamp on the forum though. It's an armshell. Handspun fibers. No rope edge. Small shockmounts. Is this possibly the 3rd generation that's so debated? I found it for next to nothing.

Anyone else have any idea?
I'm fuzzy on the transitional shells. Anyone else seen this stamp?

the Zenith P stamp
These are your regular Herman Miller shells, would possibly even be a side-shell, something that Zenith Plastics never made; Zenith Prime, and another company, Summit Prime, were very big Herman Miller distributors back in the day and would ink stamp the bottom of the shells that they supplied with the circle and their name in it, exactly your stamp. Hope this helps. Cheers.

where ?
botswana.

Londonboy you've been misinformed. "....would possibly even be a side-shell, something that Zenith Plastics never made." Sorry, but with respect, that is just NOT true. It is a well known and documented fact that Zenith Plastics made side-shells. Plenty of them. To quote the book Eames Design (Neuhart, Eames, page 142): "1950-1953 Plastic Side Chair After the successful adaptation of the stamped metal armchair to plastic, the metal side chair submitted to the "Low-Cost Furniture" competition was developed by the Eames Office for production by Zenith Plastics." Also watch this film (link below) by the Eames Office about the making of fiberglass chairs. You will see many many many sideshells. But the most important thing is the credits at 8.34: "With appreciation to Century Plastics" To quote the book Charles and Ray Eames (Kirkham, Eames, page 234): "Zenith Plastics (now Century Plastics) of Gardena, California, which had used fiberglass during the war, worked in conjunction with the Eameses and with Herman Miller Furniture Company on the new project"

Also
Summit Polymers and Zenith are known to have produced fiberglass for Herman Miller, so I would lean towards these circular stamps as indicating the producer, rather the distribution company.