Possible Clues:

• Second afterthought in a letter: Abbr.
• Second afterthought
• Label for an additional afterthought
• Letters at the end of a letter, perhaps
• Epistle appendage: Abbr.
• Second afterthought, in brief
• Second letter afterthought
• Ltr. afterthought
• Later afterthought: Abbr.
• Second ltr. addition