Possible Clues:

• Stamped return env., e.g.
• Bus. letter extra
• Bus. letter attachment
• Collier's work: Abbr.
• In an env.
• Bus. letter abbr.
• 5 Placed inside: abbr.
• Collier's, for one: Abbr.
• Something in an env.
• It may be a copy of a ck.