original bertoia bird ?  

is it a stupid question ?
Maybe it sounds like a stupid question to learned professionals. I am new to this forum, obviously. On their website, Knoll state that there should be a mark on the base. But has it always been this way since the beginning of production ? Is it possible that some original bird chairs were issued unmarked ? The dozens of pages displaying original bird chairs or other designs of Harry Bertoia from apparently reputable retailers do not show any marks.

...
... the old ones don't have a brand, so if it's recognizably an older one, you can be sure that the chair is original.
I've never seen a knockoff of it, but if so it should be a rather new one.

answering myself
well there is part of the answer :
http://www.youtube.com/watch?v=ef5vqvwFhWw&list=UUzGRSClkeAqzWMQ9PBbnIxw...

thank you Onno
thank you for your answer Onno :-). The one I found looks old, but more by the upholstery than the metal itself, it's chrome. I'll try to post pictures when I get more familiar with the forum.

moreibiza
Knoll has only been stamping their name into the Bird chairs for the last ten years; older chairs are unmarked (although the fabric cover might have a Knoll label).
There are only a few knockoffs currently being made, and they're laughably bad (pictures below). No one would mistake them for the real thing.
As far as I know, there are no vintage knockoffs of the Bird chair. As onno said, if your chair is obviously old, it's genuine Knoll.

Thank you Fastfwd, I post a...
Thank you Fastfwd, I post a few pictures of this chair for the experts to appreciate, I went back to have another look at it this morning, took the fabric cover off to examine the finishings, and this is what I saw :
- I do not remember to have seen a rail like that on the few originals I saw
- The welding doesn't seem very good even to an amateur like me

Looks genuine to me.
But in rough shape, with some surface rust and what looks to be at least one torn shockmount... Which would also explain the bad welding that you've seen: If the chair's been stressed enough to tear a shockmount, it's likely that some of the spot welds between wires (or the wires themselves) have broken and been sloppily repaired by some yahoo with a stick welder.

That's a Knoll version for...
That's a Knoll version for sure.
Its likely built after the 80's. I don't know for sure when the cups, that craddle the shock mounts, came along? Older versions don't have the cups. Maybe one of the parts people know? special k or the other guys that do come here might know.
Some china hack, tried a slighty better (relative to the repro pictures above) repro a few years back. That version looked like they copied a broken down old 50s version chair! I laughed at how it appeared to be 6-10 inches off the normal profile, just looking at the side view of it. The bird can turn in to a recliner if its all broken down.
The chair looks like it sat outside or somewhere pretty damp to pick up the rust. It might wire wheel away, with a Dremmel running a mini brush.

and the rail ?
thank you donsof and fastfwd. So this chair could be an original ? What about this rail running on the edge of the chair and holding the wires is this the original design ? It is surprinsingly difficult to find good detailed pictures of original Knoll products on the internet, I haven't found any without cover...

last pictures
Well, the chair seems to have the right shape, somehow (first picture). On the pictures of the chinese fake the shape isn't even right.
I am still intrigued by this rail (second picture). I had never seen any diamond without a cover before, or didn't pay attention because I didn't think I would one day have to judge the authenticity of one of them... So is this rail always been on the chair or is it a later addition ?
And about the dimensions, have they changed since the 1950's or have they always been the dimensions given by Knoll nowadays ?
Thanks to all of you helping 🙂

Rail
Are you referring to the double-wire edge in your first photo? The part I've circled in red below?
If so, there's nothing wrong with it. My Knoll chair has the same edge.

yes !
Yes this is exactly what I was talking about Fastfwd. I was really wondering. Now should there be any marking on the cover ?

Dimensions
The overall dimensions haven't changed, but I believe the shape is a little different now. I haven't looked at a vintage chair side-by-side with a new one, but it seems to me that old chairs have a slightly narrower "neck" where the backrest meets the Diamond-shaped seat; new chairs are thicker there. It's not a very noticeable difference, though, and new covers fit old chairs just fine.

Cover
The cover might or might not have a Knoll label. My new one does and my old one didn't.