Plate blocks, also known as plate-number blocks, refer to stamps that are still attached to their original sheet and including the serial number of the printing plate in the sheet’s margin or selvage. The format of the plate-block numbers depends on when the stamps were printed, and who did the printing. Prior to 1894, when the U.S. Bureau of Engraving and Printing (BEP) began printing stamps, private companies were contracted for this task. These included Rawdon, Wright, Hatch & Edson of New York and Toppan, Carpenter, Casilear & Co. of Philadelphia, both of whom joined other printers in 1879 to form the American Bank Note Company, which also printed U.S. postage stamps.

When it came to numbering their plates, each printer did things a bit differently. Rawdon, which printed the 5-cent Franklin (Scott #1) and 10-cent Washington (Scott #2) between 1847 and 1851 did not use plate-block numbers at all. Toppan, which produced stamps from 1851 to 1861, took a straightforward approach, numbering the first plate used to produce each run of stamps in a particular denomination as “1,” the second as “2,” etc. The National Bank Note Company, which operated from 1861 to 1872, numbered its plates sequentially regardless of denomination. National began the plate count anew for its two-color Pictorial stamps of 1869 and 1870, as well as for its Bank Note issues of 1870 and 1871.

While the National Pictorials used the same numbers for each of the plates needed to produce each color on its stamps, subsequent printers sometimes gave their plates different n...umbers to differentiate the source plate for each color. In 1883, the American Bank Note Company added letter prefixes to its plate numbers, while BEP plates were given a different number for each color beginning in 1901 with its Pan American issues, one for the stamp’s frame and another for its vignette.