5310-00-161-0528 (FFW100)
NSN 5310-00-161-0528 (NIIN 00-161-0528) is a lock washer in Federal Supply Class 5310, Nuts And Washers — a washer, usually round, with a centrally-located hole. Manufacturer part number FFW100 (Federal Specifications Promulgated, CAGE 81348); also referenced as MS45904-67 (Military Standards).
Key specs: 40 to 50 rockwell c hardness; full characteristics table below. NIIN assigned August 1972; DEMIL A (none required).