The copper plating is applied during manufacture to all the finished surfaces of the ball except the actual spherical surface. This acts as a mask to enable surface chemical hardening of the spherical surface without making the stem brittle. I worked for the Foden truck company in the '70s and they were still making their own front axles in house then including the tie rod ends. I asked about the copper plating and was told the above. They used a process called Nitriding (not sure it's spelt like that).
David
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