U.S. Stamp Notes — By John M. Hotchner
Flat plate printing was used by United States Post Office Department contractors and by the Bureau of Engraving and Printing (after it took over U.S. stamp printing in 1894) exclusively until 1915. At that point, a gradual switch to rotary press printing began, starting with the coil issues of that year.
Rotary press operation allowed for much more rapid and efficient production because the press permitted continuous printing on a base consisting of a roll of paper (called a web) that moved through the press.
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This was ideal for coil production, eliminating the need to paste flat plate sheets together in order to fabricate coil rolls.
It was not until 1923 that sheet stamps were intentionally produced by the rotary press. Rotary-produced sheet stamps before that (see Scott 538-546) were made from coil waste, meaning parts of the rotary coil web that were excess to coil production.
One of the asked questions I receive most often is “How do you tell the difference between flat plate and rotary plate stamps?”
The Scott Specialized Catalogue of United States Stamps and Covers is a great reference.
Here is what the Scott U.S. Specialized catalog says about this problem in a note that appears between Scott 447 and 448: “The Rotary Press Stamps are printed from plates that are curved to fit around a cylinder. This curvature produces stamps that are slightly larger, either horizontally or vertically, than those printed from flat plates. Designs of stamps from flat plates measure about 18½-19 millimeters wide by 22mm high.
“When the impressions are placed sidewise on the curved plates the designs are 19½-20mm wide; when they are placed vertically the designs are 22½ to 23mm high … ”
If you have a plate-number piece, such as the pair on the postcard shown nearby, no measurement is needed because the unique number for each plate can tell you if you have a flat or rotary plate.
Every ½¢ plate with a number 19650 or higher is a rotary plate that produced Scott 653. The flat plate versions, Scott 551, all have plate numbers from 17017 to 17086.