Leopold Vieyra

Leopold Vieyra (source image)
On 23 May 1916, MI6 Rotterdam station chief Tinsley reported to MI5 that the Dutchman Leopold Vieyra alias, Leo Pickard, was a German spy. His correspondence was checked by the censor, leading to Vieyra and his handlers exchanging telegrams wherein they complained to each other about the delay of their mail. Inspection of Vieyra's letters by the censor, however, did not prove the use of secret ink. Or did it?