How to tell an authentic Nelson Coconut chair  

Hi all,
1st time poster here, love the forums.
I'm looking at a (somewhat) vintage Nelson Chair. I haven't driven out to inspect it yet, but the sellers say they bought it from a licensed Herman Miller dealer in Australia. The think it's Herman Miller, but say it has no markings or stickers. Should there be any???
I know it's only available in black leather now from Herman Miller, but I see Vitra still makes them in "Orange Hopsack" which this appears to be. Proportions look right to my eye (no overly-large-radius front legs). Plus they live in a fancy part of town, so chances are good it's real... although you never know these days!
Any tips on how to tell authenticity would be greatly appreciated!
Thanks for your help in advance.
Ash