|Carl Hans Lody, WWI Germany spy.|
The letter was recently included in a display at the Guards Museum commemorating the 100th anniversary of Lody's death. The display also apparently includes the bullet which ended Lody's life as well as a sixpence that he gave to one of the guards as a tip.
The Daily Mail article also includes an inset piece on the bloody history of the Tower, highlighting some of the more famous (infamous?) executions which have taken place over the centuries. The inset mentions that during World War II, the Tower served as a "temporary prison" and served as the execution site for Josef Jakobs.
This line may not be entirely accurate. The Tower was considered to be a fortress and a royal palace during World War II, but not a prison (either temporary or permanent). While it was true that the Tower served to hold some individuals (e.g. Rudolf Hess), the Secretary of State never declared the Tower of London to be a military prison nor a detention barracks during World War II.
On the other hand, the cell used to hold Josef at HMP Wandsworth, was declared to be a military prison on 19 July, 1941 by Herbert Morrison, one of His Majesty's Principal Secretaries of State. Such a decree was never issued for the Tower of London during World War II.