My take is this :
The .15 centers were printed at then end of the US run. They were intended for sale in Canada, which would make them a Canadian price variant not a Canadian version like the later publications from National and Simcoe, which had no ads and different content.
They are so rare because there probably weren't many printed and - "In the fall of 1941, waste paper shortages occurred in Canada because laborers who normally cut pulpwood were enlisting for military service. Fearing a shutdown of the pulp and paper industry, National War Services informed the public and voluntary salvage committees about the critical need."