Ewh, ewh, ewh, I would step away from any top-secret World War I documents you may come across if I were you. Seems during that time the MI6 were experimenting with semen as an invisible ink. Oh come on, I wouldn’t make this up, it’s all there in Walte Kirke’s diary. He was deputy head of military intelligence at GHQ France. He mentions that in 1915, Mansfield Cumming (no pun intended), the first chief of the Secret Intelligence Service, was investigating the use of man juice as the ideal invisible ink. Seems it all went downhill when the agent who discovered that semen would not react to iodine vapor became the butt of jokes. Oh and it also didn’t help that the “ink” friggin stank.