Historical clue formulations

• Stamp on an envelope [and 16-Across flipped] — 2020-11-25
• Communication means since 1911 — 2018-12-28
• Letters sent by plane — 2015-03-30
• Fast delivery — 2013-05-03
• Kind of stamp — 1995-03-26
• Skywriting? — 1994-03-18