Trivia: The final chess match between Moriarty and Holmes is based loosely on a famous chess match between chess masters Bent Larsen and Tigran Petrosian. The match involved the sacrifice of a queen and a surprise checkmate, thus mirroring Holmes' apparent sacrifice of himself to stop Moriarty.
Trivia: After being cast in this film as Professor Moriarty, Jared Harris was asked to re-dub Moriarty's dialog from the original film, which was previously provided by an unknown actor. This new version of the audio is often used in television broadcasts of the first film.
Trivia: The note left by Holmes and read by Watson; "Come at once if convenient - If inconvenient come all the same." was taken directly from the short story "The Creeping Man" found in "The Case-Book of Sherlock Holmes".
Trivia: Toward the beginning of the film, Watson enters Holmes' room on Baker Street that's full of eccentricities; looking out the window is a mannequin to act as Holmes' doppelganger. This idea was first used in the short story "The Empty Room" found in "The Return of Sherlock Holmes". The dummy was used again in the short story "The Mazarin Stone" found in "The Case-Book of Sherlock Holmes".
Trivia: The plot of the movie closely follows the short story "The Final Problem", which is included in "The Memoirs of Sherlock Holmes".
Chosen answer: That's the key code for his fortune concealed in his library. The one Mrs Watson uses to relay to Inspector Lestrade. Like the book reference numbers in a public library.