Historical clue formulations

• Collector of letters: Abbr. — 2020-07-14
• Mail ctr. — 2015-08-01
• Mailing HQ — 2010-03-14
• Ltr. center — 2009-05-23
• Delivery notation: Abbr. — 2008-08-28
• Main mailbox locale: Abbr. — 2008-05-25
• Mail ctr. — 2008-02-13
• Mail place: Abbr. — 2006-04-05
• Mail-sorting ctr. — 2005-11-20
• Mailing ctr. — 2004-07-29
• Address abbr. — 2004-03-17
• Bldg. with boxes — 2002-05-05
• Mail place: Abbr. — 2000-05-06
• Major mail ctr. — 1996-10-11
• Mail HQ — 1994-01-03