In my opinion, this is technically an error, but I would consider it minor. Especially considering the grade, I would not assign a significant premium to it. The note without the error is only worth face value. A $10 premium is about as generous as I would go.
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BWJM, F.O.N.A.
Life Member of CPMS, RCNA, ONA, ANA, IBNS, WCS.
President, IBNS Ontario Chapter.
Treasurer, Waterloo Coin Society.
Show Chair, Cambridge Coin Show.
Fellow of the Ontario Numismatic Association.