These consisted of a piece of wood, hollowed out so that they could hold a small bottle in which a letter was placed, along with instructions for the finder to post it and a penny for a stamp.
Above the central portrait, each stamp featured a picture of a lamp that the U.S. Post Office Department identified as the "Lamp of Knowledge" but that I thought of as Aladdin's lamp because of the boy in the Arabian Nights with the magic lamp and the ring and the two genies who give him whatever he asks for.