Military Standard MS20002C4 washers are countersunk, high strength, heat-treated washers which must be used with the MS/NAS twelve point and internal wrenching bolts. The chamfer matches the radius under the bolt head. Also good for use with heavy torquing loads where ordinary steel washers would gall.
Pilots HQ responded promptly to my order. They were delivered on time and pricing was reasonable. I would order from them again. Top notch company, very professional.