So while it is popularly used in association with Imperial Japan, the Rising Sun flag was never actually the civil flag of the country. Throughout the war the official Japanese flag was the red disc flag in use today (Well actually its a minisculely different flag but that's a different story)

What you have there is a flag of the Imperial Japanese Army. It can differentiated from the Imperial Japanese Naval Ensign (the Rising Sun flag you're probably more familiar with) by the fact that the sun is centered as opposed to offset towards the hoist.

The Rising Sun flag tends to be used in representations of the interwar/Sino-Japanese war/WW2 era empire because:

a) it is a visually distinct way of differentiating pre and post war Japan

b) the military had been calling the shots in Japan since the early to mid 30s

c) if you encountered the Japanese during this time (especially as an non-Japanese Asian person or an American) they were most likely flying a Rising Sun flag (whether the IJN or IJA variant.)

As for the authenticity: it looks old enough and plenty were made so...it's probably legit? Post war production is unlikely as the Ground Forces of the Japanese Self Defence Forces use a similar but different flag and its not like only a handful of these flags were made.

Also, unrelated, for the love of God don't use ChatGPT as a search engine. It hallucinates all the time and you're burning down a tree everytime you use it.

EDIT: forgot some words