When they arrived, they found Yamamoto's sword and Admiral rank insignia (shoulder bars) missing. They have never been located to this day.
Yamamoto had been thrown clear of the plane's wreckage, his white-gloved hand grasping the hilt of his katana sword, his body still upright in his seat under a tree.
There seems to be a contradiction here - did he have his sword or not?
I'm not surprised that he removed his rank insignia if he had the opportunity - I can't imagine that the locals would treat him too well, and ritual cannibalism was still routinely practiced. As for his katana, that may have given him away as high-ranking as well, so may have been similarly disposed of.