Thailand. (Dai Nippon Airways). 1939. Air Mail envelope addressed to Yokohama, Japan bearing SG 254, 5s violet and SG 256, 15s blue (pair) tied by Bangkok GPO date stamp '21st Dec 39', endorsed 'By Air Mail' with Yokohama receiver '23rd Dec' on reverse.
Nice item paying the 35 satang rate via Dai Nippon Airways.