This cover was flown on a round-the world polar flight (sort of) by Rockwell Corp. It contains a letter describing the flight and its purpose. But I found it interesting that there was a disclaimer rubber-stamped front and back stating "Not carried on officially authorized mail flight." Would the post office have applied this or was Rockwell required to disclaim this on the cover, since they were responsible for transporting the mail in this case?
Listed on page 1608 of 5th edition of American Airmail Society Catalog. Their illustration show same marking, with no explanation.
Gut guess is the phrasing could have been better to say this cover *was* carried on this flight, but the flight was not an official airmail flight in terms of being done or contracted by any postal service. I.e., a privately carried letter requiring letter rate postage due to the USPS monopoly on letter mail.
(Add: when the airmail rate was more. I am surprised the USPS did not charge them the full airmail rate [as marked], for providing the courtesy cancels, but I don't know the contemporary details of the flight event.)
Since this was not a CAM route, it seems like Rockwell was fudging it a little by referring to this as a singular flight. It had five stops, after all.
Also, the letter mentions that the plane returned to Burbank, so the mail really only moved between there and Palm Springs, hence no need for an airmail stamp. But then, why is it back stamped in Honolulu?