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It's not too big of a deal since at this stage of the show, the writers can just explain it away by saying something like "the swords used by Buster, Kenzan, and others are much older, thus they can't utilze the power of two books the same way the SworDrivers can."

Honestly, this issue in itself reminds me of Zero One with Horobi having the Amazing Hercules ProgRise key, or Yua having the Freezing Bear key, only to use the keys for finishers rather than new forms since there wasn't any toys of them in those forms.