||Western Electric 520
Single line w/ industrial round wall enclosure - "explosion proof". Weighs about 20 pounds!
TO PREVENT IGNITION OF HAZARDOUS ATOMOSPHERES, DISCONNECT LINE WIRES AT THE PROTECTOR BEFORE OPENING HOUSING .
HOUSING MUST BE LOCKED BEFORE THE LINE WIRES ARE
They went through great lengths to make sure the works were completely surrounded by lots of cast metal to contain any possible sparks. The base plate and sides seem to be at least 1/2 - 1" thick. The hookswitch connects mechanically through a long plunger rod. The bell vibrates a lever that rotates a shaft that activates a clapper between the gongs outside the sealed area.
The dial linkage is also through a shaft. Curiously, the
fingerwheel is still on the dial inside the sealed area. The
fingerwheel outside rotates a shaft which has a lever and brass
"finger" which turns the dial (see bottom photo, center). It's
the only phone I can think of that has two fingerwheels ganged together.