Sterling W.A.S.P wings, Sterling Fifi & ATC Pin (Clutchback)

[Kimo]

The circular pin is called a Distinctive Unit Insignia. These were worn in pairs - one on each epaulet in the middle. This particular one was worn by the members of the US Army Air Force Air Transport Command which was in existence from November 30, 1942 to June 1, 1948. These are relatively common and go for quite modest sums when they actually sell. Recent sales on Ebay show a range of between $7 and $10 for an individual one, matched pairs is what collectors normally want and sales of those seem to be more in the $20 range. The one you show appears to my eye to be a later one, likely post-war.