In 1914 and 1915 the U.S. began experimenting with a slightly wider perforation to keep stamps from separating from the sheets in the post office drawers. The perforation change from 12 to 10 was gradually implemented until nearly all U.S. postage stamps were available in the perf 10 format. This included the Special Delivery and Postage Due stamps as well as the ordinary issue, the perf 10 Washington Franklins. It is interesting to note that the air mail stamps did not arrive until 1918 and were never printed perf 10. The Pan Pacific Exposition stamps hold the distinction of being the only flat plate U.S. commemorative stamps to have more than one type of perforation.

The Post Office did not recognize the change in perforation as a new variety, and apparently neither did collectors. Although nearly equal amounts of the perf 12, Scott 397-400, and perf 10, Scott 401-404, stamps were issued, the perf 10 stamps are much rarer, particularly in blocks.

Although possibly more than 8 million of the perf 10 ten cent stamps were issued, the plate block is exceptionally scarce. ( The 10¢ stamp is not illustrated here). Eight million is not a particularly small printing, dwarfing the 5¢ Norse American for example, yet nice mint copies are quite scarce. A nominally centered perf 10 ten cent Pan Pacific will bring more than 100 times the amount that a beautifully centered 5¢ Norse American will, illustrating the role the stamp collector provides in assuring specimens for future collectors. Obviously, collectors of the era saved far more of the Norse American commemoratives than the perf 10 Pan Pacific stamps.

The eleven cent Franklin was issued primarily for use in prepaying postage on parcels, but also paid the postage and insurance fee on insured parcels. The 11¢ stamp completed the Washington Franklin series from the 1¢ through the 12¢, with the 13¢ stamp not being issued until 1919. A 14¢ stamp was never issued in the Washington Franklin Series.