I think that is most likely the result of either being folded over on itself or having another note pressed up against it for a long time (like in a safety deposit box). That does not strike me as an error.
From the pattern imprinted It doesn't match the 20 on the 1914 FRN but rather I believe a 1914 $5 FRN was pressed up against it upside down. Maybe there was dirt or rust between the notes and that was the effect.
I would grade the note at Fine+ or Very Fine 20. With the signature combo of White/Mellon, should fetch around $150 if sold in auction.
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