This is the first error of this kind that I have ever seen on a 1937 $10 note. The prefix N/B was used on the 1937 $2 Gordon-Towers issue but not on the $10. This note has been graded/authenticated by BCS.
There is no Charlton catalogue number for this type of error so BCS has just designated it as a "serial number error". Interesting find.
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Arthur Richards
Contributor, Charlton Catalogue of Canadian Government Paper Money, 19th, 20th, 21st, 22nd and 29th Edition
Pricing Panel Member, Charlton Catalogue of Canadian Government Paper Money, 21st Edition 2009