5310-01-256-9731 (FF-W-100)
NSN 5310-01-256-9731 (NIIN 01-256-9731) is a lock washer in Federal Supply Class 5310, Nuts And Washers — a washer, usually round, with a centrally-located hole. Manufacturer part number FF-W-100 (Federal Specifications Promulgated, CAGE 81348); also referenced as MS51413-4 (Military Standards).
Key specs: 89 to 94 rockwell b hardness; full characteristics table below. NIIN assigned June 1987; DEMIL A (none required); unit of issue EA (each), $0.60 gov standard price; ships at 0.01 lb / 0.0005 cu ft.