English: Reading board 40 for the teleprinter message key sheet – also called Message (key) board

This device, which can be seen at The National Museum of Computing on Bletchley Park, was used for communicating the indicator for the 12-wheel Lorenz SZ40 rotor cipher machine from its introduction, until October 1942 when a more secure system was introduced.

Twelve letters were sent in clear before the enciphered message. The sliders were positioned against the letters and the numerical values for the wheel starting positions were read through apertures from the underlying sheet.