Hello - I am not sure whether you still check this post, but I might have some additional info for you. My grandfather was a US Marine and fought in the Pacific Theater during WWII. Several years ago, I found a Japanese prayer flag from a private first class Shinzo Furuya among his belongings from the war. Since then, I've been trying to track down any remaining family members so that I may return the flag to it's rightful owner. Today, I received a letter from the War Victim's Relief office in Tokyo, Japan. They have found the son of Shinzo Furuya and he lives in Yamanashi Prefecture. I thought I remembered seeing that town mentioned somewhere among the other replies to your post. I doubt my Shinzo Furuya is the same as yours - unless he was in his late 50's during the war...but maybe they are related?
If you are still checking this and are still looking for information, I would gladly share all that I have. Since I have not yet made contact with the son, I do not want to publish his name or address without his permission.