Plate numbers were put in the margins of United States sheets so that later the printer would know which sheets were printed from which plates so that plate wear and damage could be monitored. Collectors noticed the marginal numbers and began at first collecting single stamps with the selvedge attached as an adjunct to their collection. The Bureau of Engraving and Printing began printing US stamps in 1894 (before that our stamps were printed by private printers under contract so when our government took control of its own stamp printing it could be called one of the first examples of insourcing