fake barcelona chair with knoll stamp ?  

The back cushion is too tall.
 
Aunt M.

As you said, the look of the stamp is different.  The most obivious difference is that the kerning is way off---way too much space between the K and the N, for starters.  (I used a photo from the Knoll site for comparison.)

With that side-by-side comparison, I am reminded of "Chibson" guitars.  Choll?  
I suppose many factories might very well stamp their product with whatever stamp the customer wants to pay for.  But a printed Knoll textile too?  Are these really being counterfeited like that?  
 
neat article -  http://jetsetrnv8r.wordpress.com/2008/10/26/vintage-furniture-–-real-o...

I wondered about the backing fabric too with the Knoll name on it.  On high end pieces, whether furniture or clothing, lining fabric with the brand name is often--maybe almost always?--jacquard.  Meaning, the name is woven into it, not printed with ink.  I can't figure out why some of them would be backwards.  Actually, it doesn't make sense for printed fabric either, unless someone thought it made a cool pattern but it doesn't, really.  
That website frowns upon fakes so maybe that one will be gone soon.

It certainly seems odd but I would but very surprised if Firma could make such a mistake as I thought they had a good name.
The red one they have has the same stamp as the correct comparison stamp and states it's from the 80s.
I actually think they have their dates mixed up as the sus one looks older and the closer one looks more likely 90s.
http://www.firmalondon.com/furniture/seating/barcelona-chair-mies-van-de...
The below one says 70s and shows jacquard fabric but no stamp as they had none in the 70s.
http://www.firmalondon.com/furniture/seating/barcelona-chair-mies-van-de...
ActualliyI was correct and years mixed up on 1st dibs.Their website says white one is few years old and the image looks much better on website than the one posted here as the original sus one.
http://www.firmalondon.com/furniture/seating/mies-van-der-rohe-barcelona...
I would assumee all is above board by Firma as expected.

comparison pics - the fake barcelona and my knoll barcelona
This is the first time i see a knock off with stamps and this is not good at all as some people may considered it authentic and spend a big amount of money on it ..

Object: clearly these are being counterfeited this well. And I imagine they are being counterfeited even better. 

Starline, I agree with you that that's the same white Barcelona chair pictured on both Firma's website and in their 1stDibs listing, but where are you seeing the photo of the incised mark on their website?  I don't see a photo of the mark at all.
And now I see another difference:  the fabric on Vangelis's chair is different than the fabric on Firma's white chair.  On the white chair, the upside-down lines alternate with the rightside-up lines and all lines are equidistant from each other.  On Vangelis's chair, the lines are in groups of three:  two upside-down lines and one rightside-up, with slightly more space between each group of three.  And judging from the button placement, the fabric panel on the white chair is smaller overall.

here is a better pic of the cushion .

Oops---i was confused by the way the straps covered some of the words in the first photo of Vangelis's!  Sorry about that.  The straps kind of covered entire rows.  So the spacing and direction of the rows is the same.  My bad.

Spanky I am unsure but this same signature is on the black one they also say is a recent production as below.
Rather than speculate it's only fair I email them and ask.

Starline--that is a good idea to ask Knoll.  Though they don't have a link to anyone on their site to go to to verify authenticity!  
The photo you most recently posted has actually been photohopped!  Look how the letters extend past the lower edge of the leg.  I suppose they did it because the mark wasn't showing up clearly in the photo (chrome being difficult to photograph).  Or maybe the mark was stamped partly off the leg?? 
Another difference:  the mark in question is closer to the end of the leg than the one shown on chairs on the Knoll site.  
While I agree that it's always a good idea to go to the source, I think it's helpful to do photo comparisons here if only to inspire DA readers (lurkers too) to look closely and compare.  That's the first step in determining differences.
 
(Edited to add: the photo in the bottom half of of the one that I just posted is from the Knoll International site, in case there's any question that marks are different on chairs sold outside of the US --   http://www.knoll-int.com/home/by-designer/classic-designers/ludwig-mies-... )

The radius at the end of the leg is very different between the two chairs. A detail which I just noticed courtesy of spanky's side-by-side.
edit: the baseline also shifts up at the beginning of the signature on the bad mark, and the keening, as noted already, is way, way off. 

Well, the correct logo actually is composed of two different fonts (we used to call them "faces"), while the bogus one is not. Is it possible that this change is enough to slip by the copyright cops ?
At one time you could buy repro early-Ford hubcaps, with the curly Ford logo actually spelled "Fool" . . .