Stamped Air Mail Etiquette?

Not sure what to call the winged stamped double ring U. S. Air Mail indicator. Doesn't look printed, and there is no other directive, so who applied it? Guessing that if the cover weighed 1 oz or less, the 6c prexie tells the RPO clerk to send airmail? No markings on back.

Not sure what to call it either. Applied by sender. Possibly a rubber stamp. Not unique but not common.

Typical rubber stamp of the period used by sender to indicate airmail service.

Thank you. Here's a link to GregAlex's Feb 2017 post of a cover from the possible supplier of such handstamps. Nice looking handstamp.

https://goscf.com/t/27335&whichpage=3#462943

The marking was one that had been applied to air mail letters and cards by post offices during the first half of the 1920s as a standard rubber stamped direction. Finding it on a 1940 cover is simply philatelic. Likely it was applied not by the post office but by the sender who likely got his hands on an old post office rubber stamp. In 1940 the rate for an airmail letter was 6 cents. While most people used airmail stamps for this there was no requirement that airmail stamps had to be used and regular stamps could be used instead, which is the case for this cover. My guess is that this cover was mailed in Willits, California and since there was no airmail terminal there it was sent by rail to the nearest city where it could be loaded onto an aircraft to be forwarded to Cincinnati.