Ref. book set

• S.A.S.E., e.g
• Business letter abbreviation
• Stamped return env., e.g.
• Business-letter footnote
• End-of-letter letters
• Abbr. at the end of a business letter
• A set of bks
• Accompanier of a letter inside an env
• Letter inclusion (Abbr.)

Wiktionary Word definitions in Wiktionary
abbr. enclosure (qualifier indicating that a letter is accompanied by further material English)