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I can find references in which all the elements between the cathode and final anode are called grids. Also, notes to the effect that there were some electron gun designs using a low voltage on the focus electrode and other designs using a high voltage.
So, I could speculate that when the high voltage focus electrode was introduced, someone called it an anode becuse of its positive voltage, and then it necessarily became the first anode; but the references I have do not make it clear.
Edit: note also the term "ultor" applied to the final anode.
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