John Galsworthy is supposed to have said or written in 1920 that Looney's "Shakespeare" Identified was "The best detective story I have ever read."

Does anybody have any idea where he said or wrote that? So far the earliest source I've been able to find is Miller's 1975 reprint edition of Looney, p. 648, but she gives no source. I've searched every book review of Looney's book I can find, plus all the book reviews written by Galsworthy as well as his other essays, but nowhere can I find any mention of the Oxford theory by him.