Hello Charlie,
This isn't going to be from a Shūsui plane since none were in production.
I think it would be impossible to identify the aircraft. I don't think the Japanese put model names on their planes. (But you are correct, it is indeed 秋, meaning Autumn). Hard to say what the significance is. Pilot's name? 秋 is a common character found in surnames. Is it painted on, or is it carved/engraved into the metal?
To identify the plane, I think you'd need to have a part number and then you might be able to figure it out. OR, you could research Iwo Jima and find out what were the most common planes that were used around February 1945 there, and maybe work back from there, but even that would just be guesswork. Unfortunately the word "秋" (aki) alone doesn't provide much clue.
You might also try posting to the WarRelics Forum (in the Japan sub forum), where the guys are hyper-focused on WW2 memorabilia.