Looks suspicious. Its just a big patriotic phrase (the ever-present, yet generic phrase "Good luck in battle") and then signatures around a red circle. Addressed to Mr. Ōtake. There are no defining characteristics of authentic war flags. No location name, no name of a school or group which might be associated with the addressee, no date. Its just a flag full of names, which, tends to suggest its a flag that somebody knocked off well after the war.