Air Post stamps. 1938, Airplanes and Birds, imperforated plate proofs of 1c-$1, set of ten in pairs, each stamp with ''Proof'' overprint in black, less 20c (not issued in such format), but including two of 4c in orange (issued color) and yellow, perfect condition, full OG, NH, VF and very scarce, Est. $400-$500 (Image)