Third line at the bottom: Abbr.
Else clues• Afterthought #3: Abbr.
• Long letter extender
• Third addendum to a letter
• Third afterthought abbr.
• Third afterthought in a letter: Abbr.
• Third afterthought in a missive: Abbr.
• Third afterthought, in a ltr.
• Thought after an after-afterthought: Abbr.