You are correct that images fall into the public domain, in Canada, fifty years after the photographer's death, and he died in 1976.
But all Canadian images published prior to January 1, 1949, are public domain. I uploaded some of Knox's images, making sure I only uploaded images published prior to 1949-01-01.
Your edit to his wikidata - should I be concerned it could lead to the public domain images I uploaded being challenged?
https://www.wikidata.org/w/index.php?title=Q104217330&diff=1326594628&oldid=1325398287