I Was Today Years Old When . . .

I was today years old when I learned that

Australia's 3p and 6p airmail stamps did not pay an airmail postage rate. They were actually used to pay the 3p per half ounce surcharge for domestic airmail. As such, you should never find the stamp as a solo franking on a cover. It would be used in conjunction with a regular domestic mail stamp. This 3p surcharge existed into the 1960s.

Australia, Scott Nr C1 (1929)


Australia, Scott Nr C2 (1931)

And, to show that USPS is well in tune with itself, it had stopped inscribing Air Mail stamps with "Air Mail" long before then -- opting for the airplane silhouette. But if you look further back, you'll see anachronisms (or just plain "each department do their own thing"):

1935 -- airmail stamps now inscribed "AIR MAIL" (note 1934 Special Delivery Airmail was inscribed "AIR MAIL")
1973 -- airmail stamps now inscribed "AIRMAIL"/"Airmail"
1999 -- airmail stamps no longer inscribed, but airplane silhouette used to indicate stamp intended for international airmail service

Of course, Scott uses the section header "AIR POST STAMPS", but in the listings/notes will call them "airmail"!

Scott = consistent about being inconsistent