In the image below are two covers from the first airmail flights from Washington, DC on May 15 and 16, 1918. If I'm reading my new (to me) 5th Edition of the American Air Mail Catalog, the top cover is AAMC #101 (a). Thus it would have been carried on Lt. Boyle's ill-fated May 15 flight (which "crashed" just outside of Washington, DC, when Boyle got lost and put down in a field, damaging his prop). Covers on Boyle's flight were returned to Washington and carried again the next day by Lt. Edgerton. It is backstamped "New York, N.Y. REC'D STA. W, May 16, 4 PM 1918.

The bottom cover appears to me to be a curious example of AAMC #101(i). In addition to the mail from Boyle's May 15 flight, the AAMC says
Quote:
This mail and 1,300 other pieces which were picked up from the special red, white, and blue letter boxes after the regular air-mail collection was flown on May 16th by Lt. J.C. Edgerton to Philadelphia. These additional pieces were canceled May 16,
1918 and some have "9 A.M" or "10 A.M. 1918" in the handstamped cancellation; perhaps a dozen or more are known with "May 16 - First Trip" in the cancellation.
This (bottom) cover appears to be one of the scarce (?) covers with the "May 16 - First Trip" cancellation. But it lacks postage for the $0.24 air mail rate. It is backstamped "REC'D PHILADELPHIA, PA - May 16, 3 PM, 1918" and came (to me) with a PSE certification that reads
Quote:
United States, Scot No. U436b, flown entire, 5/16/1918,
AAMC No. 101B5, genuine, stamp missing, toned.
The certificate is dated 9/24/2010. I'm wondering if there ever was a stamp. The U436b is cancelled, and I see no evidence (residual gum) that a stamp was ever attached. But it was not marked postage due when received in Philadelphia.
What are the odds that it never had a stamp?
Basil