Appreciate the printed text: "M. C." under, and aligned with the right of, the signature: this is an abbreviation for: Member of Congress.
The Cooper Collection of U.S. Postal History
Русский: «Франкирование Конгресса», отпечатанное на конверте для конгрессмена Томаса С. Батлера (Пенсильвания). Машинный штемпель: Вашингтон, округ Колумбия, 18 октября 1921. Из Куперовской коллекции истории почты США.
This United States postage stamp was published prior to 1978 and is therefore in the public domain in the United States. See Commons:Hirtle chart for further explanation. Note that it may still be copyrighted in jurisdictions that do not apply the rule of the shorter term for US works (depending on the date of the author's death), such as Canada (50 p.m.a.), Mainland China (50 p.m.a., not Hong Kong or Macao), Germany (70 p.m.a.), Mexico (100 p.m.a.), Switzerland (70 p.m.a.), and other countries with individual treaties.