User blog comment:B214/Volume 17 spoilers./@comment-59.96.208.81-20140223141644/@comment-16281343-20140223161505

But there's no guarantee that those swords are present in the DxD Universe.

Gram, Nothung, and Balmung are the same swords but they are treated as diffrent swords here. So, we can't really count on those. What we considered are those that are mentioned in the story.