@Dabaha yeah I thought the same at first too, but I feel like anatomically it wouldn't make sense, although something like an anus/rectum might make more sense
With Kikai it's the same to me like with Kitoh Mohiro. I am really fascinated with the worlds they create and the diverse stories, but why the hell are they both so fixated on gore loli porn??
Also the "none would open it" should probably be "none could open it" (i.e. she's not lucky, she's intentionally keeping her treasure - also evidenced by the final omake).