A Back Cover Advertisement on an Article Reprint
The National Geographic Society has a long history of reprinting popular articles from the Geographic. As a rule, these reprints are devoid of advertisements, the back covers are blank. So far, I have only found one exception to this rule: the November 1936 article reprint "Trains of Today and Tomorrow". There is an advertisement on the back cover of that reprint by the Association of American Railroads:
The ad was specifically made for this reprint. The back cover ad for the original issue is a color photo of a glazed ham. Yum.