Historical clue formulations

• Coal industry org. — 2019-09-29
• Affiliate of the A.F.L.-C.I.O. — 2013-04-21
• Coal diggers' org. — 2012-01-03
• Diggers' org. — 2009-05-28
• Underground grp. — 2004-06-06
• Underground org. — 2003-06-11