I suspect the answer is the cachet maker who want to sell envelopes, in this case ArtCraft. A storied company that finally ceased producing cachets a few years ago. They started in the 1920s and had a heck of a run.
It is actually a first day cover of the stamp. The stamp was issued as a definitive stamp at a time of frequent rate increases. The new rate was not yet approved, so it was unknown what the rate would be. It ended up being an 18c stamp. The "C" stamp was issued 7 months later as a 20c stamp under similar conditions.
Roy
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"BuckaCover.com - 80,000 covers priced 60c to $1.50- 10,000+ new covers coming Tuesday June 1"
I just can't help it - while indeed beautifully executed, the monochrome design and the dark clouds in particular look more than a little mournful to me. Of course, a rate increase is not exactly a cause for celebrations, at least not for the customers ...