Pitney Bowes Model M metering machine part
In 1920 Congress passed legislation that allowed first-class mail to be carried without postage stamps affixed. This soon opened the door to metered mail.
A Post Office Department official introduced Arthur Pitney and Walter Bowes to each other because it appeared that their individual products would work together—Mr. Bowes's machine and Mr. Pitney's meter. Bowes’s stepson, Walter Wheeler, Jr., joined the design team and helped develop the final design of the 1920 model. The company’s model M postage meter was authorized on September 1, 1920, and placed into commercial use on November 16, 1920.
The Pitney Bowes Postage Meter Company manufactured the early meters. The metered stamp closely resembled permit stamps of the time, and it can be identified by the small meter number (M.1, M.2, etc.) reading between the frank and the town mark. The similarity of appearance between permit stamps and the meter stamps doomed the design to a short life of sixteen months and twelve days. Postal employees had difficulty distinguishing them from one another, which they needed to do because they handled metered mail differently from ordinary permit mail. The design was replaced by an oval design. Only sixty-nine meters with the first design were placed into use. The meter had two register doors, the top one used by the Post Office Department to set the meter and the bottom to show the total amount used.
American Society of Mechanical Engineers. "Pitney Bowes Model M Postage Meter 1920: An International Historic Mechanical Engineering Landmark." September 1986.