5310-01-258-4635 (FF-W-84)
NSN 5310-01-258-4635 (NIIN 01-258-4635) 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-84 (Federal Specifications Promulgated, CAGE 81348); also referenced as MS35338-41B (Military Standards).
Key specs: 45 to 53 rockwell c hardness; full characteristics table below. NIIN assigned July 1987; DEMIL A (none required); unit of issue EA (each), $6.33 gov standard price; ships at 0.25 lb / 0.0046 cu ft.