Flat end of the stopper goes towards the wheel.
Pull the hub off, then pipe assembly off, then slide the stopper onto the bare axles shaft, again, flat side away from the diff/ towards the wheel.
Then install the pipe housing on OVER the stopper and axle.
Pull the hub off, then pipe assembly off, then slide the stopper onto the bare axles shaft, again, flat side away from the diff/ towards the wheel.
Then install the pipe housing on OVER the stopper and axle.