Note the caption: "5 - Planet 'X" (Pluto) at last found !!!" This was likely written within a few weeks of the actual discovery (March 13, 1930) when the official name of "Pluto" was given on May 30, 1930.
The name of Pluto was suggested by Venetia Burney — an eleven-year-old English schoolgirl — in keeping the tradition of naming after gods from classical occidental mythology. And it did not hurt that the symbol for Pluto (♇) looks a lot like P. Lowell's initials, "PL".
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