"The Double-Cross System, or XX System, was a World War II
anti-espionage and deception operation of the British military
intelligence arm, MI5. Nazi agents in Britain… were captured,
turned themselves in or simply announced themselves and were
then used by the British to broadcast mainly disinformation to
their Nazi controllers." —Wikipedia

The XX —

IMAGE- 'Double Cross' flag and armband of Chaplin's 'Great Dictator'

The Double Cross of Fritz Leiber—

Source: Rudolf Koch, The Book of Signs