Background
The numbering of printing plates has long been a part of quality control in the printing process. That way, if someone at the printing plant notices a problem with the printing of a certain stamp, the plate number can be used to locate the proper plate or cylinder so the problem can be investigated.
In some cases, for instance the Penny Reds of Great Britain and modern United States plate number coils, the plate numbers appear in the stamps themselves, but the more common practice is to include the number in the margin of each sheet, sometimes alongside the name of the printer.
On coil stamps (stamps issued in a long band of single stamps with the edges imperforate) a plate number sometimes is printed on the margin of a stamp, which collectors refer to as a plate number coil. Mint plate number coil stamps are most often collected as strips of three or five with the stamp with the plate number at the center of the strip.
Read more about this topic: Plate Block
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