"See attached" letters, in a letter

• Something in a ltr
• Collier's work: Abbr.
• "Look inside" abbr
• Compton's, for one: Abbr
• Bottom-of-letter letters
• Check included with a ltr., e.g
• What's inside an env
• Insertion with a letter (Abbr.)
• Business-letter abbr

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