Possible Clues:

• City addr. info
• City address abbr
• Abbr. in many urban addresses
• Part of a st. address
• Part of many an urban addr
• 221B, e.g
• Metropolitan address abbr
• Abbr. in many an urban address
• The super's is usually 1A: Abbr
• 1-A or 3-B, perh.