this is something thats puzzled me for awhile and now has popped up on ebay!
I un-upholstered two of these exact same chairs last summer and realized they have a very tapered back. I did some searching to see if there was any special name for them with no luck. I've seen other uncovered chairs that had the regular swoop, did the covered chairs just start to change shape in later years to ease the covering process?
http://www.ebay.com/itm/Vintage-Mid-Century-Modern-Eames-Shell-Chair-Retro-Herman-Miller-Swivel-/300756619722?pt=Antiques_Furniture&hash=item46067dd1ca
what you have here
looks like the european/vitra version of the armchair, which was different from the Miller production from the beginning.
But I've also had an upholstered Miller chair like this from 1989 with the european form but I never figured out why, when, or for how long Herman Miller changed their design (or why vitra went with a modified design to begin with)...
here's a direct comparison:
If you need any help, please contact us at – info@designaddict.com