Historical clue formulations

• Tough nut to crack — 2024-08-22
• Cipher machine of W.W. II — 2024-07-23
• Riddle — 2024-02-12
• Very secretive sort — 2022-09-07
• Personality that's hard to read — 2021-10-24
• Monthly publication of the National Puzzlers' League, with "The" — 2021-08-15
• Conundrum — 2020-11-30
• Puzzle — 2020-09-30
• Tough nut to crack — 2020-05-29
• Mystery — 2018-02-04
• The Mona Lisa's smile, e.g. — 2017-10-21
• Conundrum — 2017-03-26
• Tough nut to crack — 2016-10-22
• Nazi cipher machine broken by the Allies — 2015-11-18
• Mind-boggler — 2015-08-19
• "The Imitation Game" machine — 2015-07-17
• Riddle — 2014-09-23
• "I'm a walking, talking ___": Larry David — 2012-02-18
• Appropriately named monthly of the National Puzzlers' League, with "The" — 2010-02-21
• Head-scratcher — 2010-01-30
• Riddle — 2009-11-24
• Riddle — 2009-04-06
• Knot — 2007-09-01
• Conundrum — 2007-06-05
• Riddle — 2007-04-10
• Head-scratcher — 2007-03-04
• It's not clear — 2005-12-11
• It's hard to understand — 2005-02-19
• "Hmm?" inducer — 2003-07-20
• Stumper — 2003-02-15
• Mind-boggler — 2002-11-08
• Poser — 2002-08-01
• Riddle — 2002-03-12
• Stumper — 2001-11-21
• It's mind-boggling — 2001-10-25
• Perplexer — 2001-10-15
• Something inexplicable — 2001-06-26
• Mystery — 1999-10-11
• Complete baffler — 1999-07-14
• Mystery — 1998-04-14
• Elgar's "___ Variations" — 1997-12-24
• Poser — 1997-10-19
• Sphinx — 1997-08-03
• Puzzle — 1997-05-20
• Elgar's "____ Variations" — 1995-08-28
• Perplexity — 1995-07-13
• Puzzle — 1995-05-02
• Mystery — 1993-12-31