Some Early 20th century US Airmail Postmarks

[Butterfly]

While going over some US airmail stamps from the 1920s to 1940s I was awed by the variety of cancels.  So here are a few examples along with some discussion.
 Please feel free to add info or examples to improve this very basic show & tell.
The most common type of cancel in this era is called a duplex cancel and consists of a dial or postmark section on the left and a killer section on the right which usually goes over the stamp. The year is displayed either in the dial or between the dial and the killer (It is called a year slug).

Here are a few killer/dial combos.  Different numbers or things can appear in the middle of the killer and I don't have a clue what they signify.  One killer also had vertical lines.
 





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[Butterfly]

The top stamp shows a particularly clear date slug
The second stamp has a simple hand cancel of a type often seen on parcels.
The third stamp was stamped twice and the date is only a smudge at the lower left and no inner circle.(see below!)
The fourth stamp has some other type of duplex, with only a few horizontal bars for the killer.

EDIT: i goofed and transposed the third stamp with the first stamp of the first post.

[Butterfly]

These stamps were issued later and show some different killer designs.
The bottom one appears to have GPO in the center of the killer.

[Butterfly]

Finishing off with a few "air mail saves time" machine slogan cancels

[kacyds]

Great looking cancels Butterflies, esp. those slogan ones. Thanks for sharing!!!!

[MikeC99]

Butterflies, I checked my collection and I could only find two airmail postmarks.  This first one is Scott # C23 issued on May 18, 1938.  It has a beautiful air mail cancel like yours above.

[MikeC99]

 This stamp is Scott # 858 issued on  Nov. 2, 1939.  I love this great air mail cancel.

[JanetC]

This is a really good topic. Very good cancels on all of them.