Eames Zenith Armshells...1st gen  

I have heard of two options for cleaning zinc / chrome-plated bases:
A. Aluminum foil and white vinegar.
B. Brasso, a popular metal polish.
This blog shows an example of the Brasso in effect on chrome legs. http://eamesrestorationproject.tumblr.com/post/50705909270/eames-dcm-walnut
This blog shows an example of foil & vinegar on chrome bodies.
http://plastolux.com/refinish-modern-chair-eames.html#.V17y4eYrJTY
Hopefully this helps.
Best,
Burbank

Beautiful chairs! Congratulations
It is often the case that the fiberglass shell seats show a slightly different character than the undersides-- especially after 65 years. It is most likely from normal repeated use.
If the chairs were outside for an extended period of time, the wear and difference in shine would probably be much more severe.
Rather than thinking about the evidence of use and age as a problem to be solved, I would suggest that you might embrace that aspect as one of many signs of authenticity.
Well-seasoned shells have a character that cannot be faked, and over the years I have come to enjoy the way each shell has a different character and a different life in the world.
There are many products that you could put on the shell seats to try and create a more perfect match, but I would be very careful before applying anything that would permanently change the surface. I have found that cleaning with mild detergents will not harm the fiberglass, and after awhile, much of the sheen can be restored with repeated cleanings.
The shells never had a "coating" to begin with.
The shine to the undersides is simply the well preserved original surface of the mint condition fiberglass. So to apply a product like Penetrol to these rare and beautiful shells would destroy their vintage character, in my opinion.
Go easy on 'em. As they are only original vintage shells if you do not re coat them.
Hope this helps!