In Ben Macintyre’s Operation Mincemeat How a Dead Man and a Bizarre Plan Fooled the Nazis and Assured an Allied Victory he details:
“the top secret Double-X committee or twenty committee was the group in charge of overseeing the exploitation of the double agents [during WWII], so called because the two roman numerals formed a pleasing pun in the form of a double cross. The name may have also been an ironic tribute to Charlie Chaplin who’s A Great Dictator, a film released in 1940, features a dictator operating under an ‘XX’ flag, mimicking a swastika.
“The 20 committee existed during WWII and met every Thursday to discuss the Double-Agent system run by TAR Robertson, exploring new deception plans and plot how to pass the most usefully damaging information to the enemy. Its members included representatives from Navy, Army and Air Intelligence as well as MI5 and MI6.”
So, if the Double-X committee represented Double Agents and was established prior to Special Training School 103 (STS-103) and the purpose of STS-103 was to train agents, then a logical deduction would be that a graduate of STS-103 would become a Double-X operative. If so, then clearly, as a student, you’re starting as an inductee… not a double “X” yet, but only a single “X.”