One side of this nut is machined flat to go against the subframe body where it holds the brake hose in place. There is a shakeproof washer
WE600101 fitted first against the machined side. On the assembly line the factory fitted this nut on the left hand side and a
21A228 ordinary nut on the right hand side where it also goes against the washer. The right hand nut
21A228 has been superseded by this nut which has an identical thread and diameter.