I don't have much new information about the flying post offices other than to say it was a short-lived experiment conducted after WW II. Here is some selected information and images I posted on another forum:
This is a philatelic cover used three years after the trial period. I bought this cover for its Walter Crosby cachet depicting the interior of a flying post office and the photo (Crosby's signature piece) of mail being loaded onto a flying post office.
As a former Railway Mail Service substitute distribution clerk, I was interested in the flying post office experiments conducted in 1946. The Post Office Department conducted these experiments in late September and the first week of October of 1946. Covers for these flights are readily available on eBay
at low cost. I have, however, only one of them at this time. It is almost identical to yours and depicts a Fairchild Packet (C119 Flying Boxcar) which was used on one of the routes.
The experiment was not continued as it was quickly discovered that airplanes did not provide the enroute mail sorting efficiency that trains and ships did. The flights were too short in duration to sort much mail and it was difficult for clerks to move around while the aircraft was in flight. All-in-all, a short lived trial.
This photo is from the National Air & Space museum of PO employees sorting mail while in route on one of the flights.
Note the pistol the clerk on the left is wearing. He was most likely the clerk-in-charge and responsible for any registered mail on the flight. Clerks were required to be armed while handling or escorting registered mail at the ends of the runs. Indeed, when I signed on as a Railway Mail Clerk, I was issued a snub-nosed .38 special revolver and a box of bullets. I remember wearing it when delivering registered mail from our mail car to a RMS transfer clerk.