Original or not  

Are you referring to the shells?
Or are you looking for an original RAR rocker?
The shells in the photos are all original. The white/parchment one looks to be in the best condition. The ochre (or yellow?) one is a nice color, but condition is difficult to tell from the small photo.

Original or not
Thank's so much for your answer. Yes I'm locking for a RAR Rocker. The white one was my favourite but as I found no HM mark on it, I was not quite sure. The seller says that it is a 1955 one with the Z for Zenith, but I read that they made it until 1953! Is there another way to be sure (for example on the shockmounts). I try to add the last picture I have, the one with the "Z" that shows also the fiberglass. Once again many thanks and sorry for my english!
http://s1211.photobucket.com/albums/cc438/Kat033/

Rocker
Original rocker bases can be somewhat difficult to find in good condition. Good luck in your search.
The white shell is authentic Herman Miller though.

buyer beware
I believe the early Eames fiberglass chairs by Zenith Plastics were never produced with an embossed "Z" but only with a square red white black decal that would state that the chair was produced by Zenith Plastics for Herman Miller in Gardena CA. Also a very early fiberglass armshells will have an unusually large I mean larger than the usual rubber shock mounts and the edges would have what is known as a rope edge, literally a rope was imbedded along the edges of the chair with the fiberglass molding process. The earliest RAR were never sold to the public but was given as gifts to Herman Miller employees having a baby.
Make sure that if you are paying top dollar for this chair that the seller will guarantee that they are selling you the real thing and if it turns out to be a fake that you can return it an get your money back and put the transaction in writing. Most refutable dealers will have no problem with this but if this dealer says no way, walk away fast.
Pay accordingly, there are vintage later fiberglass shells being sold attached to a more recent reproduction base and if the base is vintage, the armshell might be more recent. Remember there are companies out there like Modernica in LA that still make new fiberglass shells.
Also check the Eames Office Gallery website and go through the vintage Eameas furniture raisone section and look through the images of what early versions of this chair should look like. ( http://www.eamesoffice.com/eames-furniture-raisonne) Good Luck!

Original or not
Thanks for your answer Minimoma. My God it is so complicated! The seller is a private seller. He told me that the chair has little shockmounts due to the fact that it was produced in 1955, after the first generation (these with the rope). What seems quite unusual is that it has no Hermann Miller Logo on the fiberglass shell. The rockers are new. He wants 1500 $ for this chair. What do you think, is it the price for such a chair? Can I trust him ? In the Eames Raisonne there is no mention about a RAR production in 1955. As you can see on the pictures, the white one (told as a 1955) has an ink mark and a "Z", the red one (also told as a 1955) only a "Z" and the ocker one (without mention of the year) a ink number, a HM mark and a paper with numbers on it. Why these differences? For the first two he asks about 1500 $ and for the ocker one "only" 1200 $. Is it due to the colour? All of them have small shockmounts! With many thanks in advance for your very appreciated answers.

Hi
The shell is nice, but not worth near the asking price. Arm shells in good condition are going for about $300 - $400 in the current market. A new reproduction rocker base costs around $100 or less.
Edit: Where are you located? The prices I stated above are for the US. I expect there is a significant mark-up in Europe and other regions.

Yow! Those prices are REALLY ...
Yow! Those prices are REALLY high. You can buy an armshell on ebay (with a boring base) for $300-$400 easily, and then just get a repro rocker base (as this guy did) for under $100 on ebay too.
So you can basically get a chair as good as this one for under $500. In my opinion, this guy is trying to fleece you...

Ha...you beat me to it Woody!
Ha...you beat me to it Woody!

Original or not
In Switzerland! Here everything is "cheap" 😉 What about marks on the shells? Why are they all different as the years are the same?

The first two shells
Might be referred to as "3rd generation"... Zenith production early to mid-1950s, no HM embossment, smaller shockmounts with holes in them.
The third shell has a patent sticker and is probably a Cincinnati Milacron production (will have a "C" embossed). These started in the late 1950s.

Hi vogo,
in Switzerland...
Hi vogo,
in Switzerland you can find Miller RAR for 1400-1800 CHF with ORIGINAL (not like in your photo) base by professionnal dealer.
For a 1st gen Zenith rope edge with original base = 3000-3800 CHF.
2nd gen zenith = 2200-2600 Chf.
"z" zenith Herman miller 3rd gen= same as for a H.Miller.
You find the shop under links on this website.. good luck..

Original or not
I'd like to thank you all VERY MUCH for the useful and complete explanations you gave me. I learned a lot and I'm now ready (let's say almost ready) to find my RAR which I'm sure, somewhere - expects only me. Once again many thanks and best regards from Switzerland.

original or not
Prices for a vintage Eames RAR varies depending on location and demand and more recently economic climate so check around your local market first before committing to this one.
Also to clarify when you say "Z" embossed mark, did you mean the old Herman Miller embossed mark, if it is, Zenith versions will not have any but just the decal I mentioned earlier and early Herman Miller will have it and later ones will spell it out. Also Herman Miller is a stickler to tags with dates of manufacture, especially patents, flammability requirements etc. so the absence of these is a sign of that it was removed to make dating the piece difficult for someone who is not familiar.
The Zenith versions from very early 1950 only came out with the first base which is called the x-base with screw on glides. Also if examined carefully, the rubber shock mounts will be a good tell tale sign of what base was attached to the shells because each of the base(s) X, H, rocker, effiel tower, cat's cradle will leave a very distinct mark on the rubber after sitting on the chair for so many years, if it does not match, the base was replaced.
It also used to be a very distinct characteristic of the Zenith shells is their translucency to light, later Herman Miller versions are opaque and light does not pass through. Modernica in LA new versions actually copied this but fortunately these are embossed with the Modernica logo. Remember, Zenith versions have big rubber shock mounts, rope edge, translucent and decal (if not missing) so pay accordingly.