They seem to be a lot of bank statements, a bunch of certificates of nonsense business ventures or even actual legit ones from Back In The Day involving some shenanigans in the post-War era. But probably out of place is a death certificate with no doctor's signature for a one Ushiromiya Kinzo dated for November 29th, 1984.
Picking that one up gives the knowledge: the head of the Ushiromiya family died unexpectedly and his death was covered up. For two years, conspirators pretended he was still alive so as to put off the matter of inheritance. The secret of his death was suspected by the three other siblings who weren't involved with the cover-up, and they had planned to confront their eldest brother, but it all went awry when a letter from the Golden Witch appeared at dinner declaring that she had reclaimed what had been loaned to the Ushiromiya family, but if they could also the riddle of the epitaph, they could earn the title of head of the Ushiromiya family as well as the tons of gold Kinzo was rumored to have hidden on the island.
Re: Rokkenjima
Picking that one up gives the knowledge: the head of the Ushiromiya family died unexpectedly and his death was covered up. For two years, conspirators pretended he was still alive so as to put off the matter of inheritance. The secret of his death was suspected by the three other siblings who weren't involved with the cover-up, and they had planned to confront their eldest brother, but it all went awry when a letter from the Golden Witch appeared at dinner declaring that she had reclaimed what had been loaned to the Ushiromiya family, but if they could also the riddle of the epitaph, they could earn the title of head of the Ushiromiya family as well as the tons of gold Kinzo was rumored to have hidden on the island.