One of two blue air letter cards used by Union Castle Line. There is a picture of the steamship RMS Windsor Castle next to the Union Castle Line with the words "AN AIR LETTER SHOULD NOT CONTAIN ANY ENCLOSURE , IF IT DOES IT WILL BE SUPERCHARGED OR SENT BY ORIGINAL MAIL, THE APSLEY AIR LETTER". The letter comprises of three folded sections, one of which is blank. The second section contains the steamship, flag and lines for filling in the sender's name and address. The third section has 5 lines and in the top left corner the words "BY AIRMAIL AIR LETTER, PAR AVION AEROGRAMME".