Well, bear in mind that the fakes we've seen seem to be contemporary with the time - I mean, they were apparently souvenirs brought over from Japan immediately after the war. Maybe some enterprising Japanese war veteran was making them in his garage. Maybe some Chinese guys saw a business opportunity and began churning them out. Who knows?
I wouldn't completely rule out the possibility that it is something more than a souvenir. The obvious fakes we've seen had things like Tojo Hideki's "signature" on them, and/or all of the writing on the flag is obviously done by the same hand. Some of the writing on your flag looks like it was done by one hand (the bottom left quadrant), but in other places the writing is different. And you don't have any obviously dumb phrases on the flag (like "World War 2" or some other phrase that would be out of place on a Japanese flag). So, I can't say for sure what your flag is, but I would have to say it looks a bit suspicious.
There is also the name of some principal (can't read the name) on the flag.
One of the phrases in the bottom left: 見敵心殺 I've never seen before. Google is telling me it means "search and destroy".
Edit: Another odd phrase on there 討伐奸敵 - basically destroy all enemies, I guess, but it is as uncommon as the one above. I've never seen either of these phrases on a Japanese flag. Starting to think it may be a Chinese counterfeit.
Wondering if it could be a flag given to a kid upon joining a sports team, and the flag has been doctored up to look like a war souvenir?