(Historical note — written while the payoff was still forward. Modeled on the Urashima Tarō tamatebako — opening it ages the opener — an explicit fairy-tale overlay. The age-altering property has now paid off on-page @0650; see the confirmed callout above.)
Earliest known caller
(unrecorded)
Why this matters
Oda plants a named, specific prop with a single defined power inside the very setting whose fairy-tale (the dragon-palace / Urashima Tarō legend) is the arc’s mythic skeleton. A prop-tell + fairy-tale-overlay seed: when a one-use treasure is named and its exact effect spelled out, it is loaded for later. Cross-link Vander Decken IX, Shirahoshi, Tamatebox.