Are all authentic Barcelona chairs stamped?  

Knoll has been stamping the Mies signature
only since 1996.
A little more info is here:
http://www.designaddict.com/design_addict/forums/index.cfm/fuseaction/th...

Sorry to get back..
this late on your question. Two details indicate to me that these are not produced by Knoll. The Knoll Barcelaona chairs do not have these clearly rounded ends on the feet. In order to make sure that the crossing of the two blades does not reveal which of the blades is welded and which is continous, the radii on the inside are quite generous. In this case they seem to be smaller than the originals.

Koen is spot on.
Plus, the cushions don't seem to fit properly. A snappy enclosed of one of my vintage barcelona chair legs.
Best,
Mark

The
Same thing with the backrest. Also the round edges and the whole curve seems to be wrong. On the original one at the top the curve turns almost straight again. this one is too wide apart from the back. I would pay some attention to the sofa table though...

Good to see the Barcelona...
Good to see the Barcelona questions.
Maybe somebody can also tell, if the metal on the backrest is divided under the leather straps?
I mean, I have seen some Barcelonas, where there is under the first (or 2nd ?) strap from the backrest if you put it away a bit, the metalparts are divided and put together.
Is is original, or also an indicate for a fake ?

There was some discussion about this some time ago.
As I recall, the Knoll chrome plated version offered by Design Within Reach has a pieced together frame. The stainless version is one piece.
Best,
Mark