[Kaoru Chujo]

I don't think the mangaka was very careful about chronology. But I think we are supposed to feel as if it is post-WW2.

The policeman's wish that the tank had been finished before the war ended makes me think readers are supposed to think of WW2. The big airplane makes me think of that period, too. And TV broadcasts were not common at all, even in the US, until the late 1940s. The cars look like anything from the 1930s to the early 1960s to me (in Japanese cars), but not earlier, I don't think. The horrid portrayal of China is ahistorical Japanese fantasy, in my opinion, and shouldn't be taken seriously. But the idea that Japanese troops had been in that area previously suggests a period after WW2.

The captain appears to be European, and his wish to fight the nation that produced Nijuu-mensou doesn't necessarily imply any knowledge of Japan's post-war constitution. The comparatively nice lifestyle was probably just fantasy, but I think that although the big business groups were split up after WW2, the Americans let many of the people who ran them carry on, in order to keep Japan's economy going, partly to help in the Korean War. And there were still lots of small submarines in the 1950s: the US apparently had over 200 diesel subs when the war ended, for example (Wikipedia).

But all this is beside the point if I am right and the mangaka was not being at all careful about his chronology. I think we are just being given a retro-feeling background.

As for the bombs, maybe we are again in "just believable enough" territory. I'm not sure that explosives left in the plane for years would still work -- nor that a plane under development would have been booby-trapped like that -- but I also can't see how the Senchou could have carried enough or gone enough places to booby-trap the plane as widely as it exploded. So I'm not looking for what would really happen so much as what the mangaka wants us to think. I do believe we are supposed to think crafty Nijuumensou planted the explosives in some nigiri, and that Chiko was crafty enough to come up with a clever answer. But why her answer was clever is still not clear to me: because she was lying, or because she was so adult in her way of expressing the captain's mistake?

I guess Nijuumensou gave the captain half the gold to get him to cause no more trouble and not call out his men for a fight that could have damaged the sub and done everyone harm, and whose outcome no-one could have been sure of.