Plate block
USA, 1895 issue
A plate block is a block of postage stamps that includes the part of the sheet margin with the serial number of the printing plate(s) used to print the stamps.
Sometimes the plate numbers appear in the stamps themselves, but the more common practice is to include the number in the margin of each sheet, frequently alongside the name of the printer.
Plate Block Media
A US plate block from 1972. Note the addition of a number for each color. This increased the cost of collecting plate blocks. That, plus the format change, discouraged many collectors.
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