Historical clue formulations

• Apt example of this puzzle's theme — 2015-07-15
• "Puts the keys of the future at your fingertips" (Philadelphia, 1876) — 2012-07-29
• With 49-Across, where 17-, 24- and 39-Across are seen — 2007-02-07
• Place to find keys — 1999-03-19
• End of the explanation — 1994-06-10