Related Clues:

• Cover-letter letters
• Something inside an envelope: Abbr.
• Compton's, for one: Abbr.
• SASE, often
• Letter-bottom abbr.
• An SASE is one
• Env. in an env., say
• Env. insert
• Letter-bottom letters
• Env. inside an env., e.g.
• It's in an env.
• Ref. wd.
• Something sent with a ltr.
• Env. attachment
• Check, perhaps: Abbr.