I think maybe some early collector put that card on the left in a folder or book or scrapbook that had pages with a low ph, and that affected the back of the card, causing the brownness. Similar to how many books printed in the 30's with lesser quality paper that continue to deteriorate due to the acidic content.
Or maybe its backside got slightly scorched once upon a time.
The E121's I've seen/had/have don't appear to suggest different cardstock.
Last edited by FrankWakefield; 12-13-2020 at 09:35 PM.
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