On top of those Brits, it seems that "Joan Clark" was used as a pseudonym in the 1930s by, of all people, Mildred Benson, who helped write the Nancy Drew series! (Benson was almost 30 years older than Clark.)

https://www.goodreads.com/author/show/5415434.Mildred_Benson

Under that name, Benson wrote the Penny Nichols mysteries.

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