CQ, CQDX are not the only uses for these letters. In the early 1900s, 'CQD' 'Come Quickly, Distress' was used as the maritime distress call and 'SOS' 'Save Our Souls' hadn't been fully adopted when the Titanic incident occurred.

Titanic actually used 'CQD' and 'SOS' as she was sinking, as the radio operator must have been aware of the new signal, although it wasn't guranteed that any ship receiving it would understand the 'SOS' message.