Hmm... does indeed look as though Ripperberger did not pass beyond the veil until 1974:
Darn, looks as if I haven't done my homework properly on this one. Isn't copyright legislation fun...
However, did the Messrs. Huebsch and Ripperberger translate this work for hire? In that case, doesn't the copyright belong to the publisher, and corporate copyright expires 50/70 years after publication.
I found the copyright for the 1943 U.S. edition assigned to Viking, does that indicate that the translation is a corporate copyright there, in any case?