Anyone know what this chair is? Junk? Mies? Knockoff?  

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Looks like Mies to me, what do you mean about the curve?
At one point Knoll reinforced the major bend with internal rods, so maybe that would account for the extra weight...or they just changed tube gauge for a while.
*jealous*

For the curves, I was...
For the curves, I was referring to how I knew it wasn't the Spoleto - those legs are straighter than Mies' design.
While it looks so much like Mies, I can't find anything that has that weird metal spring suspension. All of the Mies I see uses nylon strings.
I'm quite confused!

Mies didn't design the Spolet...
Mies didn't design the Spoleto, sorry I think we are at cross purposes and I just woke up 🙂
Mies however did design the chair above for the Stuttgart housing exhibition in 1926 I think? MR10.
Maybe its a copy, I don't know, was the label just a tag from the shop or a proper Knoll tag?
No I'm wrong, he came up with the semicircular curve after seeing Stams chair, there was lots of fighting about this...I'm not at home so can't refer to any books about it sorry, yes there is coffee in me now and I see your point.

Sorry, I am not...
Sorry, I am not communicating clearly!
What I was trying to say was that the store had a little printed out tag saying it was a Spoleto chair by Breuer. I knew that it wasn't, as the shape is wrong. To me, as it does to you, it looks like a Mies design.
The only thing I am not sure about is the metal coil springs used as the seat suspension. I am not at all knowledgeable about Mies, but I cannot seem to find any images of his chair using coiled spring suspension. They all seem to use nylon or leather string, as in the attached image.
If I am incorrect and this is a Mies chair, I will be absolutely thrilled!

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I have a feeling its the proper thing, maybe you could email Knoll about the springs though I'm sure someone on here will know about it.
Ufficio Tecnico Spoleto?
http://www.knoll.com/products/product.jsp?prod_id=54

Hmm
I've seen them with springs before, but I don't know if they are original to the chair.

I guess the next question...
I guess the next question is, does knoll sell the replacement leather individually?
I shot them an email so we will see what they have to say about the whole thing.

If you go to a Saddler for le...
If you go to a Saddler for leather, buy a Stanley knife, a long steel rule, a hole punch and some eyelets you could probably do the whole thing yourself for not much.
Good luck!

I'm guessing
That you got this chair for real cheap. I would suggest just redoing it yourself with some nice bulk leather upholstery.

I think I found two things...
I think I found two things which may tell us if it's a knockoff or not. I'm actually suspecting it is because the welds are very visible.
1. On the back of the frame near where the leather of the backrest ends, there is one bump on each side of the frame (intended to hold the leather up I assume). It's almost like a metal nipple welded onto the frame.
2. There are visible welds under the leather of the seat.
Is this the case on genuine models?

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Hows about the magnet test, did Knoll use stainless on these ?

Hi there,
 
I know a  while has passed but wondered if you were able to get new seats for your chairs? I need to relace some on thrift store finds of my own.
 
Thanks,
 
KG

All authentic versions of this chair have leather laces on the back and only Thonet produced the chair with stitches on the sides of the back , i believe that all other versions with springs are not authentic ..