Paasche Airbrush catalog #18 from 1923. This page features the  "Models D & E." The "D" & "E" are double action versions of the "H" & "F." Just as with most double action airbrushes you pull the trigger back to control the paint. Pulling back on the trigger actuated a rod that moved the "color adjusting part" in and out of the air payh.Unlike most double action airbrushes these are external mix.  Paasche didn't have any internal mix airbrushes at this time, they were all external mix.