Is this an original Barcelona chair or not?  

dude you buy one, and i'll buy the others!
they superficially look correct.
How much are they?
they certainly look of an older vintage and look very much like the ones i've seen with proper labels. can you take an underneath picture?
Dude we gotta get em! come on i'll buy lunch..........

hi, here an additional...
hi, here an additional picture (sorry it's everything I have from the underneath of the cushion..).
what seems strange to me is that the chair have 9 straps instead of 8 as it should. And normally between every leather squares you have a leather roll, but here the leather is simply folded (it is one big leather part for the whole cushion instead of small leather parts for each square)...

Not Real
My inclination is to lean towards a very good copy. The Knoll chair has 9 straps along the seat and 8 along the back, but I can't imagine a Knoll chair would forego the welting on the seams. Other indicators are no tufting on the backs of the cushions. The back is usually covered in leather. Your chair looks pretty good in that respect, but the welting is problematic. Perhaps check with Knoll to see if they ever sewed it as such.

erzsek,
Your photo's are not that great, but I'd say that your cushions are not from Knoll. The cushion attachment is wrong, in addition to the lack of welting. But the stainless frame looks somewhat correct. If you'd send a photo of how the straps connect to the frame...especially the screws....I'd be happy to compare it to my older Knoll barc's, and report back.
Best,
Mark.

Check with Knoll?
Hi there,
Would it be an idea to check with Knoll themselves: www.knoll.com?
http://www.barcelonachairs.net