After Ducky mentions that Major Maguire's eye was found in his stomach, Gibbs asked if he also found a toe. This refers to when Ducky found a toe in the stomach of Charles Bright in Smoked (2006).
Ducky explain that SOS replaced the existing distress signal CQD and that CQD stand for Come Quick Distress. Like SOS, CQD does not stand for anything. Come Quick Danger, Come Quick Drowning and Come Quick Distress are backronyms, new phrases constructed specifically to fit an already existing acronym.
Ducky (David McCallum) explains how SOS replaced CQD as the international distress signal. In the movie A Night to Remember (1958) McCallum played Titanic's Assistant Wireless Operator Harold Bride.