Hello,
The flag is full of patriotic slogans, some with signatures placed after them. One of the slogans also has a date next to it: Shōwa 19 (1944), June 6th. The intended recipient would probably be the largish name on the right side of the flag, which I think is 幸田光造 (Yukita Mitsuzō - other pronunciations are also possible). Another slogan has the name of a location next to it: Chikuzen Haki-machi, Soto Sōzu (筑前杷木町寒水), which is near the middle of the southernmost island of Kyushu.
So there are a few interesting things with the flag: Names, location, date, and its kind of unusual to see a handful of people, each signing with a slogan. At first I thought the flag was trying too hard to be a wartime artifact (i.e. someone really worked hard to create a believable fake). But now I've changed my mind and I think its probably the best WW2 flag we've seen in a while.
Caveat: I am not a native speaker/writer of Japanese, and sometimes I cannot tell if, for example, there are Japanese letters written by a Chinese person (which can look slightly unusual to Japanese people). Anyway, it looks OK to my eyes, but you might want to hear a native speaker's opinion as well.