Related Clues:

• Ref. work
• Bottom-of-letter abbr.
• Multivolume ref.
• Letters in a letter
• SASE, e.g.
• Something in an env.
• Abbr. on a business letter
• Ltr. insert
• Stamped return env., e.g.
• Env. abbreviation
• Collier's, for one: Abbr.
• In an env.
• Britannica, for one: Abbr.
• Bus. letter extra
• SASE, perhaps