According the editors of the 1999 edition of the Unitrade Specialized Catalogue of Canadian Stamps approximately 400 copies of the
Seaway Comemorative were issued with inverted centres. There are fourteen recorded covers or cards with this error properly used.
This “fake” was probably intended as a prank: the unlikely placement of a regular stamp beside an invert is the first clue that this is not real.
On January 2, 2015 Antigua and Barbuda issued a souvenir sheet depicting
the famous "St. Lawrence Seaway invert" as part of its rare stamps series.
Bibliography
Bileski, Kasimir, et. al. UNITRADE Specialized Catalogue of Canadian Stamps 1999. Toronto: The Unitrade Press, 1998.
"Canada." Standard Postage Stamp Catalogue. 1997.
Molson, Kenneth M. Canada's National Aviation Museum. Ottawa: National Museum of Science and Technology, 1988.