Related Clues:

• Collectors' org.
• Hardly ever seen
• Return address letters?
• Return addressee?
• Not to be had
• Bibliophile's label
• One-in-a-million
• Far from frequent
• Controversial agcy. since 1862
• Part of a return address?
• Of interest to collectors, say
• Chophouse request
• 1040 sender
• Atypical
• Quite unusual