Nice! I would vote for it being a piece of test equipment, basically a butt-set unit designed to work on 4 wire circuits. What is inside the metal chassis?
There is also a good chance that there is a small schematic printed on a label inside said chassis, a common trait of equipment of that era. If there is, it might tell you everything you want to know.
The number stamped on the chassis is reminiscent of Western Electric numbering of that era. I don't know if Northern Telecom used similar numberings. Is there a manufacturer's logo anywhere, even possibly inside the metal chassis? (I'm dying to see inside the chassis, in case you haven't guessed...)
By the way, Welcome to The Forum!!!
Jim
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Speaking from a secure undisclosed location.
