how to spot a bertoia fake  

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can't help you with the markings, but I have seen some pretty bad fakes, and usually what confirms it is how the legs are welded to the main part. I don't know how to describe it but it is a pretty big difference. Then again, you might be talking about the bertoia fakes that have the exact same joinings in which case my advice is totally useless - goodluck!

if you are not in a rush
Customer Service at Knoll can authenticate chairs if you send them a couple of good jpgs, if you don't mind a bit of a wait for a response. They've helped me ID a couple of things in the past and are very courteous and professional. Authentication just takes a little more time.

The fake diamond chairs I have seen...
The fake diamond chairs I have seen all had very obvious differences with the real one. you can recognize a lot of the fakes by looking at the way the "wires" of the chair are connected to the outer edge of the chair. Some fakes have the wires welded to the underside of the outer frame, othes have a double outer edge, with is "sandwiching" the wires. You can take a close look at the edges of a Knoll Diamond Chair at www.knoll.com. As you can see there,... the wires need to be on the upper side of the edge. I also noticed a lot of differences in the way the base of the chair looks.
Hope this helps...
greets!

Check these
Folow the link below for the pictures on the Knoll website.
Also notice that at the back of the chair,.. where the base connects to the chair,... the metal bar points out. Some copies do not have this. I have also seen copies that do not have the horizontal bar on the backside.
http://www.knoll.com/products/mediafinder?srch=broad&priv=0&keywords=Ber...

Do you have an idea of the prices?
I have been offered two Diamond chairs bought in the 60's made by Knoll. They are finished in black with no seat pads. They are in almost as new condition with the exception of 2 welds on the back of one of the chairs. I have looked at you posting re how to spot fakes and am 100% sure thay are genuine. Do you have any idea of their value? They are being sold for £250, (which seems good to me).

Hi there Launchpadlondon ...
First of all,... I want to make clear I am not an expert. I pointed out some significant differences in diamond chairs that I have spotted by gathering as much information as I could, and -maybe even more important- examining the real chairs very carfully. Please do not conclude that the chairs are 100% certain genuine Knoll diamond chairs based only on the things I posted here. It is possible that there are fake ones out there that have less obvious differences with the real ones and that I haven 't seen yet. To be really sure, I would have to examine the chairs themselves.
I have no idea what a good price is for the chairs that have been offered to you. I bought a 50's Knoll diamond chair about a month ago. It is a chrome one with the original vinyl seat pad, and in excellent condition. I paid 350 euros for it, witch is approx. the same price as the ones you talk about are being sold for. Do they ask 250 for the two? Or 250 a piece? If you can get them both for 250, I personally would take the risk, if they match my earlier description. If they ask 250 a piece,... you can always mail me pictures of the chairs if you want. I can see if I can tell you something more about them.
greets,
goOve