About what did Darnay tell everyone that seemed to affect Dr. Manette? Who noticed this? What might Dr. Manette’s reaction to this story mean? (Book 2 Chapters 1-6)

After dinner, Mr. Darnay comes to call. Dr. Manette is in good humor until he gets flustered when Darnay tells a story about the Tower of London, in which many prisoners' initials were carved. The only ones that couldn't be matched by a former prisoner were D.I.G., which the guards figured was an imperative to dig (they dug, but found only remains of a possible letter). (ch 6)