Zenith or not ?? (stamped 1 ZENITH C.A)  

My understanding is that...
My understanding is that this chair is from the last production of the armshells made by Zenith Plastics. The first generation of the Zenith chairs have the rope edge and big shockmounts, the 2nd generation has only the big shockmounts, the 3rd generation has small shockmounts and the embossed 'Z'just like your chair.
I think that there was also a last transitional generation of the armshells before the HM logo was introduced, which had no markings at all. I'm not sure if these were made by Zenith also.
I have 4 lemon yellow armshells with the same marking as your chair. They all have a large amount of visible fibers, just like the early Zenith production chairs.
By the way, your rocker base is a reproduction, but you knew that already I guess?

Thanks...
Thanks alexandersforum,
You sure solve my problem with this arm chair.
Now i know it Original Herman Miller arm chair.
It almost weight and look the same as the Herman Miller arm chair.
I got a pair of this shell few month ago though a friend in USA.
Very much confuse me when it reach into my hands.
First of all there is not sign of common familiar marking on the shell itself.
Just a embossed 'Z' with 3 small dot. Try to search online by just cant find
any form of link to this arm chair. So, i stop search about this for month
till i find this forum.
By the way. Zenith Plastics are from C.A (California)?
Thanks again.
Ya, the base is from Modernica L.A.
I have a few waiting to be fitted with VINTAGE EAMES fiber shell. : )
Cheers,

Yes I think they were based...
Yes I think they were based in Gardena, California.

Now i have a better picture...
Now i have a better picture on what the black stamp has a C.A below the Zenith name. Problem all solve! And thanks to you alexandersforum. *Bow*
Cheers!

...just happy that my nerd...
...just happy that my nerd knowledge can be useful to someone...