In a striking affront to probability, up from The Great Pile rises a similar/later product from the Folkard Company of Canada Ltd, Drummond Building, Montreal, Canada, with an image copyrighted 1929.
This one is edged with WAG (Water-Activated Gum) for ease-of-sealing before posting, and perforated on three sides for ease-of-opening after arrival.
I think our common purpose would be better served by calling these admittedly post-card-like products Folding Letter Sheets
, to wit:https://en.wikipedia.org/wiki/Letter_sheet
... yes, the word "folding" is redundant
Most of the post-card-like-folding-letter-sheets I've seen bore identifying information that suggests that they were placed in hotel/motel rooms for guests' correspondence.
I suspect that these never caught-on because they did not age well, sitting in hotel/motel desk drawers, waiting months to be used ... it would not have taken too many guests, traipsing to the front desk for a larger envelope or some tape, to bring the re-order rate down to zero.
/s/ ikeyPikey (who would only like to live long enough to ever see more of his post-card-like-folding-letter-sheets emerge from The Great Pile)