Blackmail and Holmes/Moriarty

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I'm looking for a short fic I read ages ago, possibly on Cox and Co. Years after Reichenbach, Holmes is approached by a blackmailer who acquired Moriarty's estate at auction. Among Moriarty's mundane academic books, said bad guy finds a love letter from Holmes to Moriarty (the two apparently had an affair while the former was a student). Any help finding this would be greatly appreciated!