15 PK Coupling nut used with Military Humvee headlight switch MS 51113-1.

Nut, Coupling, For Electric Light Switch, 1.75 X 18
NSN 5975-00-771-6634, 5975007716634.  Coupling nut used in conjunction with military wiring harness on such items as Military Humvee headlight switch MS 51113-1.