LEXINGTON — Like the other Minutemen in his company, Bill Poole will grab his musket, sling his cartridge box over his shoulder, and stride onto Lexington Green this morning to fight, and lose, the famed first skirmish between Patriot and Redcoat. But unlike his comrades in the annual reenactment, Poole will carry with him a piece of a 235-year-old mystery that still surrounds that momentous clash: the question of who fired the shot that sparked the opening volley of the Revolutionary War. Poole, 76, is the direct descendant of Ebenezer Locke, a man who, according to one account, fired the...