Toronto. Remember film? The exposed film had to be processed so the results (good, bad, or indifferent) could be viewed. For years Kodachrome was purchased with processing included, but along the line the American government decreed Kodak had to separate the film cost and the processing cost so other companies could process slide film.
Kodak then joined the fray by offering the prepaid mailers so American customers could still get guaranteed Kodak processing for both Kodachrome and Ektachrome slides. As far as I can recall, in Canada, we always sent Kodachrome back to Kodak for processing since the mailer was included with the film price. I rarely used Ektachrome which being a simpler process like Agfachrome could be processed locally.
The above ad appeared on page 72 in the August 26, 1966 issue of LIFE magazine. My heartfelt thanks goes out to fellow PHSC member George Dunbar for sharing his research with us.