Possible Clues:

• Ltr. accompaniers
• Things with ltrs.
• SASEs, for instance
• S.A.S.E.'s, e.g
• Bus. cards in commercial mailings, e.g
• Cover ltr. accompaniers
• Docs. sent with the letter
• Bottom-of-letter abbr.
• Env. contents besides letters
• What SASEs are