This is a kind of Flat Washers.
Part Number : NAS1149
Function : Washers Aircraft ( NAS1149 Series, Replaces AN960 Series )
Manufacturers : Genuine Aircraft Hardware
Description :
1. Remove all burrs and sharp edges.
2. Dimensions in inches and apply after finish unless otherwise specified.
3. Surface roughness shall be 125 microinches Ra per asme B46.1