Historical clue formulations

• Certain something — 2025-01-25
• Je ne sais quoi — 2020-06-13
• Mystery quality ... or what 18- and 55-Across and 3- and 32-Down have? — 2009-10-26
• Hard-to-define influence — 2009-03-13
• Unknown — 2006-10-01
• Unknown element — 2006-05-28
• Unknown element — 2004-05-02