From Takeda Yoshitaka via PM Press:
This is a picture of the newly-discovered letter and its envelope. It looks like a blank piece of paper, but it actually contains a hidden message. (Photographed by Takeda Yoshitaka)
Kanno used a needle to inscribe the hidden message on the paper.
Kanno Suga (aka. Kanno Sugako, 1881-1911) was the only female among the twelve socialists who were executed in the High Treason Incident of 1910. A letter Kanno sent from jail to a journalist has been discovered in Abiko City, Chiba, Japan. In the letter, Kanno insists upon the innocence of Kotoku Shusui (1881-1911), who was one of the other twelve who were executed. The letter looks blank, but it actually contains a hidden message. In order to avoid censorship, Kanno poked tiny holes in the letter, thereby inscribing a message on it. The message cannot be read unless the letter is held against the light. This year, 2010, happens to be the 100th anniversary of the High Treason Incident. Therefore experts are surprised by the fact that such a significant primary source remained to be discovered for as long as 100 years.