I think you are referring to the a part that looks like
the dust cap at the right of the upper diagram on this page. (ignore the rest of that drawing - it is not showing the torque-tube bell/ball cover you are referring to)
Post a Parts Wanted Ad in the Swap meet. Maybe someone knows where the part can be purchased but I have not seen it sold as a reproduction part. I have seen/bought a reproduction ball/bell and it did not come with that dust seal (threaded collar "nut").
Or, try calling Jim Carter Antique Truck Parts (or, Bowtie Bits) - they might have a used part.
Can you post a photo of what ball/bell parts you have, so that we might be able to better respond about what you are missing?
