There is some dispute regarding the origin of the design of "Fleet of Columbus". Like the previous value, it is widely attributed to an unknown Spanish engraving. However, a similar image also appeared in an American book some six months before the Exposition. There are significant differences, however, and philatelic authors researching the issue have stated that it is not possible to conclusively determine the origins of the design with the information known. The stamp itself paid the first-class rate for double-weight mail.
The most significant collectible variety in the set also occurs on this value. The normal color of this stamp is a shade known as ultramarine. A very small number of 4-cent stamps were printed in error using the wrong color ink, a significantly darker shade that more closely resembles the blue of the 1-cent stamp. At least two error sheets, totaling two hundred stamps, are thought to have been produced, although significantly fewer copies are known to have survived. The "4-cent blue" is thus considered a great rarity, regularly selling for thousands of dollars.