Historical clue formulations

• Thought after an after-afterthought: Abbr. — 2012-03-17
• Third afterthought in a missive: Abbr. — 2010-10-21
• Afterthought #3: Abbr. — 2009-03-13
• Third line at the bottom: Abbr. — 2002-05-19
• Third afterthought in a letter: Abbr. — 2002-02-12
• Third addendum to a letter — 1993-11-23