Barcelona Chair  

Current production (1995+) is signed
on the leg, not underneath. I didn't think pre-1995 chairs were signed anywhere. Plus, that logo looks pretty sloppy; I guess it could be real -- I'm no Knoll logo expert -- but it looks different from the Knoll Studio logos I've seen on other pieces. Look, for instance, at the capital "S" on yours and compare it to the "S" in a known-good KnollStudio logo, or compare the relative weights of the "Knoll" and "Studio" fonts in your logo with those in a known-good logo.

The "Knoll Studio" tag certainly looks authentic...
...and it also looks to have been affixed to the chair in some hasty manner - is that glue around the edges?
Metal tags are found on the undersides of table tops and on wooden chairs - and also on eBay.
I've never seen metal-on-metal like that - especially on the iconic, expensive Barcelona chair.
The chair could be real, but I'm dubious after seeing that tag.

Hmm
That is odd. I would think that the frame would be directly stamped.
Can you share more photos of the chair?

Thanks for the feedback. I...
Thanks for the feedback. I just took a couple of iPhone pics of the whole chair and posted them.
I tend to agree, it doesn't seem authentic. The dealer (who is local to me) was sure it's real, but I'm doubting his honesty now that he won't return my calls.

Did those photos show up? ...
Did those photos show up? If not link is:
http://adairfreeman.com/studio/Misc/8/caption/

So sorry.
Your Barcelona is a knock-off.
Best,
Aunt Mark

or I should have just said..
that your chair was not manufactured by Knoll.
Best,
Aunt Mark

Yeah, not manufactured by...
Yeah, not manufactured by Knoll but someone tried to make it look like it was? disgusted (mostly with myself for not realizing it immediately!)