The vertical writing on the right is a good luck message, and to the left of that is the recipient's name: Ōmori Yasuyo (some slight doubt about the given name Yasuyo, because it sounds like a girl's name, but I suppose it can be a male name also). The family name is Ōmori.
The giver of the flag is Matsubara Yosomatsu. There was a person with this name who lived in the southern part of Japan, who eventually became president, and then chairman of Hitachi Shipbuilding.
The rest of the writing is just names.
There is a group called the
Obon Society who works on repatriating WW2 items. The Japanese consulate nearest you should also be able to help repatriate this item.