"In the case that the author is anonymous or pseudonymous the term for a work is 70 years from the date of first lawful publication"
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For all practical purposes (in my opinion, at least), a translator that cannot be identified is anonymous, and one for whom no biographical data can be found is pseudonymous. So, in absence of reliable information, we just take publication date + 70 years. Even if the book had been published in 1946, we would allow it. If more data becomes available, we can change the status, of course.