The U-75/U cord plug is used to be used for military aircraft cord and most recently for US Forest Service aircraft.  (Most have converted to the TP-120 or U-174 plug.) The U-75/U mates with the U-61/U jack

Wiring Configuration:

Mike High to Pin 1

Earphones High to Pin 2 and shield

Microphone Low to Pin3(tip)  

Earphones Low to Pin 4 (sleeve)

MIL-C-22442/5-1

Other items you may need:

U-61/U Jack