Within a 12-month period,
Canada Post offered this kind of envelope on two stamp issues: the 17c Provincial & Territory Flags of June 15, 1979 and then the 17c O Canada Centenary issue of June 6, 1980.
Here is the envelope for the Flags (and showing the pane - both at the same scale):

I recall both of these envelopes simply lying on post office counters. You could take as many as you liked. There were given away free.
These are not considered presentation items.
I have made a note to include an illustration in the next Unitrade, as a footnote to the respective issues.
Robin Harris
Editor, Unitrade