ID eames chair?  

Some eames chairs did indeed...
Some eames chairs did indeed end up in libaries etc this is not at all uncommon, but the ones I have seen, have never ever had their mark sanded away, sometimes they do have an additional paper label / tag containing an asset number or something similar, but this type of tag would have been done by the institution NOT at the eames factory. So really that part of the story the dealer has told you would not explain why someone would sand thd herman miller mark. This chair and story rings alarm bells.

Modernica
It's Modernica for sure. My friend used to own a shop which carried their stuff so I saw a fair few of their shells. Not a doubt in my mind.
Avoid that dealer like the plague.

Maybe if it were left out in the SoCal sun
for six or eight months it would look more authentic?