[Milsurplus] What was "test unit" "I-85-D" made for the Signal Corps - US Army by Western E

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What was "test unit"  "I-85-D" made for the Signal Corps -  US  Army by 
Western Electric used for?
An info appreciated!
Many Thanks  Ed#  _www.smecc.org_ (http://www.smecc.org/)  
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