Identifying the US Airmail stamps is very easy.
Eight designs were printed in both sheet and coil format. Unlike some early US coil issues, none of the Airmail coil issues are rare. Fakes of Airmail coils are possible, but I’m not aware of any. A few of the line pairs on early Airmail coil stamps are a little more costly. Make sure someone hasn’t used a pen to add a joint line to an otherwise ordinary pair of coil stamps.
The sheet stamps (listed first) and their coil counterparts (listed second, which are imperforate on two sides) are:
Scott #C33 and #C37, the 5˘ DC-4 Skymaster
Scott #C39 and #C41, the 6˘ DC-4 Skymaster
Scott #C51 and #C52, the 7˘ blue Jet outline
Scott #C60 and #C61, the 7˘ red Jet outline
Scott #C64 and #C65, the Capital and Dome
Scott #C72 and #C73, the Runway of Stars
Scott #C78 and #C82, the 11˘ Jet airliner
Scott #C79 and #C83, the 13˘ Winged Envelope
There are only six Airmail stamps that require a little extra work to identify them. They are:
Scott #C12 and #C16 are the 5˘ purple Winged Globe issue. A perforation gauge helps here. Scott #C12 is perforated 11 on all sides. Scott #C16 is perforated 10.5 by 11. A quick check of the top or bottom perforations easily tells these two issues apart.
Scott #C58 and #C63 have very similar designs. This is the 15˘ Statue of Liberty issue. On Scott #C58, there is an orange frame around the left hand side of the stamp showing the Statue of Liberty in black ink. On Scott #C63, there is no orange frame line around the Statue of Liberty.
Lastly are Scott #C129 and #C132. This is the William Piper issue. On Scott #C129, there is a thin blue line of ink between the top of Piper’s hair and the top edge of the stamp. On Scott #C132, Piper’s hair contacts the top of the stamp and there is no thin blue line. Scott #C132 is worth more as a mint single and is worth considerably more in plate block form. When buying discount mint postage, you may get lucky and find some #C132 copies in there instead of the inexpensive #C129 copies.
The rest of the Airmail stamps are all design different and pose no problem in identification. That’s it. I told you it was easy!