The U.S. Army BD-71 Switchboard was used during World War II and Korean Wars by many units for field or camp communications. The devices had six line capabilities that allowed direct line communications through the use of field telephones. I restored this set to full operating condition, thanks to Steve Hilz for the spare parts!
BD-71 Manual from 1944 TM-1138 Dec 1944
BD-71 and BD-72 Instructions for use 1937 Manual