Vivaldi, Is there any sort of editor's credit on the score? Perhaps a preface or afterword in Japanese signed by an editor? It strikes me as odd that Ongaku No Tomo would go to the expense of producing a new score from the ground up without having some editorial claim.
It also might be helpful to compare the Ongaku No Tomo score with the Durand. Are there any differences? If it is a literal re-engraving of the Durand score - note for note, dynamic for dynamic, slur for slur, etc., the copyright claim is dubious. However, I'd be extremely reluctant to allow it here since it still falls under the 25-year limit we observe for retypesets.
I'm wondering if the Ongaku No Tomo score might be a reprint of a urtext edition produced in the 1970s by Peters Leipzig - which was then an agency of the DDR government. They produced new engravings of a fair number of early 20th century works, notably those of Debussy. These pre-1983 editions are PD in the EU and in Canada. Muzyka also produced new engravings of such works during this era. Some were even sent to the same Leipzig firm (Röder) used by Peters.
Go ahead and post a few pages and I'll look it over. I own several of the above-mentioned Debussy scores so a comparison will be possible.