Take, for example, these 1963 Flag Midgee cards on Bozo Gum side panel:
Bozo, you will recall, was a gumball brand introduced by Topps in the late 1940's which migrated up to Canada for a spell before reappearing in the U.S. in the 1970's (near as I can tell). Flag Midgee cards were originally a Topps issue in '63, amidst a wave of panelization that petered out quickly. Here's a regular panel as issued by Topps:
The reverse is informative:
The Bozo cards would be blank backed of course. There is also a reference on one of the Benjamin Sport Americana Guides about blank backed cards being found in cereal boxes.
There are 99 cards in the Topps set (33 panels) but as you cna see below, only 36 in the Canadian version (12 panels) and the cards have been reoriented to the vertical:
In the Topps set those cards are numbered, from top to bottom, thusly: 16, 99, 94. I'm not sure if that corresponds to the original Topps ordering or not. I'm also not sure of the full Canadian checklist as cards from that country are not was well documented as ones from the U.S. The box above is an O-Pee-Chee product, although it's hard to read in the scan. It's a neat little set with a bit of a twist, eh?