Historical clue formulations

• Closing part of an address — 2018-04-11
• Number of letters — 2016-01-15
• Letter number — 2014-08-15
• 90210, e.g. — 2011-01-03
• 90210, for Beverly Hills — 2008-01-15
• 12345, e.g. — 2006-07-13
• Delivery aid — 2004-05-14
• State follower, sometimes — 2003-02-22
• Numbers on letters — 1999-11-21
• Numbers on a letter — 1994-12-23