David,
I agree with your third hypothesis and even just finished discussing it with Richard, the restorer of the box..literaly minutes ago.
A good connection point for field telephone wire.
Maybe also for feeding 12 VDC outside the box to operate radios on the batteries inside the box.
DEFINITELY not for 110 VAC . Too much danger of electrocution i figure.
Can you image the result of shorting 110 VAC on the aluminium and steel body and everyone in contact with it ?
The high voltage low ampereage current generated by the field phone ringers his sufficent to make a man'S HAIR TURN WHITE can you just imagine imagine 110 VAc ?
Just an hypothesis again.
Nice diagram by the way. Let me be your first client.
