s64 Cesca chair: is this vintage?  

Hello all,

I saw some Thonet s64 cesca chairs advertised at a Spanish website, TokyoStory.

But I have been having some trouble determining whether they are the real thing without being to see them close up.

I have been looking through the threads here and I understand that there have been some design changes over the years.

For example, I saw a picture of one at the San Francisco museum of modern art which seems to have a more bent seat at the front and a straighter back rest at the top.

Should I be worried about the provenance of these Spanish chairs or am I just comparing to the wrong reference?

http://www.sfmoma.org/explore/collection/artwork/48050#