Charles Augustus Milverton, blackmailer of society women in the 1904 story that bears his name, is assumed by critics to be based on a real person – but which real person is open to doubt. The favourite is Charles Augustus Howell, a larger-than-life associate of the Pre-Raphaelite Brotherhood (whose members knew him as ‘Owl’), friend to James McNeill Whistler and Algernon Charles Swinburne, and one-time secretary to John Ruskin. However, it is by no means established that Howell was, in Lancelyn Green's words, a ‘scoundrel and blackmailer’. He certainly seems to have fallen out with a lot of people, but the more outlandish stories about his life and death – Oscar Wilde may be the source for the claim that Howell was found dying outside a Chelsea public house ‘with his throat cut and a ten shilling piece between his clenched teeth’ – may be urban myths rather than actual facts: his death certificate, for instance, records that he died of pneumonia. Howell's reputation as a blackmailer seems to have originated with Swinburne, who blamed Howell for pawning some of his more colourful letters; these fell into the hands of an antiquarian bookseller who then offered to reunite them with their author for a fee. Even so, Swinburne's post-mortem verdict on his former friend – that he lies ‘in that particular circle of Malebolge where the coating of eternal excrement makes it impossible to see whether the damned dog's head is or is not tonsured’ – seems a little excessive.

Another candidate identified by Lancelyn Green is Dr Charles Augustus Bynoe, a London general practitioner convicted in 1892 for forgery with intent to defraud. There are good reasons for thinking that Doyle was aware of the Bynoe case, not least as W. T. Stead, a friend and fellow Spiritualist who became Doyle's political opponent over the Boer War, had campaigned for Bynoe to be exonerated, and published a detailed investigation of the case in 1895 under the title Wanted: A Sherlock Holmes! A Chance for Amateur Detectives. The Bynoe case, at least as Stead tells it, is indeed Holmesian in its combination of elaborate deception, subtle clues which are misread by police detectives determined to convict the obvious suspect, and prosecution of an innocent victim who has been deliberately ensnared by the villains.