The air force was created in 1947. Before that, American air power was a function of the US Army, and it fell under the Army Signal Command during WWI. The National Defense Act of 1919 created aviation reserves for the first time, the Signal Officers Reserve Corps, and Signal Enlisted Reserve Corps. Greaver Miller was certified by the army as a pilot but decided to go on Reserve status, meaning that his army responsibilities were part time. He most likely had a full-time job elsewhere and made sure to keep up his certifications periodically so that if he was called to duty out of the reserves, he could serve his country. The signature of Rich Field’s Captain Whitesides can be seen at the bottom of the reserve document.