Related Clues:

• Letters at the end of some letters
• Letter-bottom abbr.
• An SASE is one
• Env. in an env., say
• Ltr. afterthought
• Second letter addendum
• Env. insert
• Letters at the end of a letter, perhaps
• Letter-bottom letters
• Label for an additional afterthought
• Second afterthought
• Ref. wd.
• It's added to an env.
• Env. add-in
• "Look inside" abbr.