Steve -
If you're referring to the small rectangular box, it
was for field looping of the dynamic brake in a multiple
unit consist of locos with "field loop" dynamic brakes
(old F-units, SD7/9). A maximum of 4 units could be
consisted for field looping the dynamic brake. There was a
switch on the back wall of the cab that had to be set on
the lead unit as to the number of units in the consist
with field looping. A three-pronged cable had to be
connected between each unit in order for the system to
work properly. Second generation power had a different
control of dynamic braking called, "potential", but still
had the rectangular box to be able to MU with the older
units, but wasn't required to have the cable connected for
a non-field loop locos in the consist.
The field loop receptacle of the 1950s is similar to MU
air hoses on modern diesel-electric locomotives (SD70MAC
and newer). The hoses are there to MU with older units
air brake systems; "modern" units MU air brake system via
in cab computer so hoses do not need to be coupled and
cut-in. Still need the 27-point control cable connected
between units for operation, though.
Bob