The box is described as being made of laquer tied with a silken cord with tassels of red silk.
Urashima asks what is inside and is told it is something precious, and that he must promise to never ever open it. He agrees.
Returning home he finds 300 years have passed and opens the box in dismay to see 3 purple wisps of cloud rising from inside.
He suddenyl grows very old from this and dies, hinting that the precious thing inside was Urashima aging process so he did not die whilst spending time with Otohime Sama.1
1. Ozaki, Y.T. (1903) “The Japanese Fairy Book”. Westminster: Archibald Constable & Co. Ltd.
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