US Regular Issues of 1922–1931
The Regular Issues of 1922–1931 were a series of 27 U.S. postage stamps issued for general everyday use by the U.S. Post Office. Unlike the definitives previously in use, which presented only a Washington or Franklin image, each of these definitive stamps depicted a different president or other subject, with Washington and Franklin each confined to a single denomination.
Source: Wikipedia — US Regular Issues of 1922–1931 (CC BY-SA 4.0)