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Though the denim jacket was first made in the late, 1800's, the phrase 'Canadian Tuxedo' actually has its source from the 1950s, when an incident occurred at a Canadian Hotel:
In 1951, Crosby and a friend were denied entrance into a Canadian hotel because they were dressed entirely in denim. When Leviβs designers caught wind of the incident, they created a custom jean tuxedo for Crosby, ensuring that he would always be able to wear his beloved Leviβs in any fine establishment.
So the source of 'Canadian Tuxedo" definitely predates the above mentioned 1973 wool shortage.
See also
Levi's Vintage Clothing Brings Back The Original "Canadian Tudo"