These are links to all words found in the canon of Sherlock Holmes stories by Arthur Conan Doyle. The full text of the stories may be found on Wikisource.
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Oddly, the only word used in the entire Canon that begins with 'x' is x itself.
From The Greek Interpreter.
Mycroft picked up the Daily News, which was lying on the side-table.
"'Anybody supplying any information to the whereabouts of a Greek gentleman named Paul Kratides, from Athens, who is unable to speak English, will be rewarded. A similar reward paid to any one giving information about a Greek lady whose first name is Sophy. X 2473.' That was in all the dailies. No answer."
I suppose the X 2473 is an anonomous reply service either provided by the newspaper or well-known enough at the time to be understood in context.