Genuine OEM Volkswagen Locknut, part number WHT000428. This nut is designed to resist loosening under vibration, ensuring a secure connection. It is identified as item 3 in the diagram, likely used in the brake master cylinder assembly. The locknut has dimensions of 0.03m x 0.03m x 0.017m. Genuine OEM locknuts are essential for safety-critical systems like brakes, ensuring they maintain their torque and prevent component separation under stress.