Maybe its a reference in the story "the turtle and the rabbit"?
I think it's that Japanese folklore where a person saved a turtle being bullied by the ocean then the turtle repaid him by bringing him to the underwater kingdom where there were a lot of pretty women. After that he was gifted with a box and sent back to the shore. The turtle said not to open the box at any cost but he did then he became an old man. Dunno the title but that's the summary anywway
If this arc have a box this might be it
But does anyone know which folk tale THIS short story is supposed to be based on??