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Reading up on the Japanese submarine force during WW2 and I came up on this little nugget.
So what if instead of only implemented the lesson very late, the Japanese implement them earlier and on larger subs.
So lets assume that designs on a new submarine begin in the middle of -40 and take about two years with construction beginning in the middle of -42 with another year until they are completed.
That would give the Japanese navy a new type of very dangerous subs by middle/late -43, though with the usual teething problems with new vessels, early -44 would be more realistic.
It wouldn't be a war-winning weapon, but it wouldn't definently cause casulties on the US Navy.