The American Naval Bombe N-530
Input letter or energiser
On the BTM Bombe, the input letter is set by the vertical column switches. The N-530 is more complex but whilst the input letter can be set on a rotary switch on the Neon Light Chassis this can also be set by plugging this output into any socket on the Diagonal Board panel
This extract from the full circuit diagram shows the energiser switch connected to the diagonal board but omits the plug and socket between this switch and the diagonal board connections (this only shows 5 diagonal wires whereas there are 26)
Shown above, an energiser socket plugged into one of the possible 26 positions on the diagonal board chassis
Above the energiser switch on the neon light chassis.
It is called a plug on the above diagram but is actually a free socket