We may be talking of two different parts. There is a molded plastic spline protector the covers the out put spline. There is also what is called the donut. It seals the spline to the lip seal. It takes the place of the where a slip yoke would be in an installation. It is used to prevent any
oil in the box from leaking out in shippment. Although these are shipped"without"
oil, there is enough residual
oil in them to make a mess on an air shippment if the donut seal is not used. It is the donut seal I was referencing.