Historical clue formulations

• Kind of code — 2023-02-17
• Kind of code — 2018-07-22
• Relating to 51-Down — 2016-09-27
• Unlike most mail nowadays — 2015-07-05
• US*S — 2015-02-03
• Part of U.S.P.S. — 2007-08-14
• Mail-related — 1996-11-25
• Kind of service — 1996-02-11