Vintage Knoll upholstery? Hardoy/butterfly chair  

Are you sure these are Knoll chairs?  The butterfly chair design was copied by many companies back in the day.  I think they might have even been sold in the Sears and Penney's mail order catalogs.
That fabric looks a lot like the plaids that were used on the American-made Danish Modern style wood frame armchairs in the 60s.  I don't know if there's anything like it on the market now, or who made it back then.  
I DO know that you need a very tough fabric with no stretch to it for these chairs.  There is a lot of stress on the fabric where the stitched pockets fit over the four points of the chair, so the fabric itself needs to be tightly woven and durable.  The stitching should be done in a heavy upholstery thread.     

I don't know for sure if they are authentic knoll or not.  I do think they are old, and they are well made and heavy.  That's all I know about them.  Are there any distinct features that I should look at to determine whether or not they are authentic knoll chairs?  Welds, measurements, or marks of any kind?  

I don't know how to ID Knoll butterfly chairs or if it even matters.  I had 11 vintage butterfly chair frames at one point (got most of them for a dollar or two each at auction because they didn't have slings).  Some are heavier gauge steel than others, some have bent tips and some have straight tips, and one or two have sloppy welds.  The dimensions vary a lot, too.  None are marked.  Except for the one or two that are relatively lightweight, I like them all and they all look pretty much the same when the slings are on them.  (I got both the cotton canvas slings and the plastic all-weather slings from Circa50.com but their service can be spotty.)
Here's the Hardoy chair in MoMA's permanent collection:
http://www.moma.org/collection/browse_results.php?criteria=O%3AAD%3AE%3A...

Thanks for all of that great info spanky!  I really like the chairs and don't care either way if they're 'authentic' Knoll or not, I just like the way they look.  Did I read correctly in another thread that you restored yours in the past with wire brush and flat black spray paint?  Anyway, thanks again!  

I may have used a wire brush but I think coarse steel wool is more efficient.  Wipe the rust dust off with alcohol on a rag if you want to paint right way, otherwise just a damp rag is ok.  The spray with Rustoleum flat black.  I once got semi-gloss black by mistake and used it anyway and it looks so wrong!
I left mine out year-round because i didn't have any good covered storage for them at the time.  The paint lasts about 3 years before needing to be redone again, unless you're okay with a more weathered look.