I suppose the GUPPY conversions (post-war) were technically begun in the 40s. Fleet boats didn't have snorkels or streamlined superstructure until then. Most of the other details remained the same though.
It should also be remembered that the American fleet boats were probably the most sophisticated submarines in service at the time, and did not reflect the general state of submarine technology in other countries, which is what a Swedish "black project" would have drawn on. The German U-boats (aside from the Type XXI) largely reflected First World War design principles, but with updated materials and construction techniques.
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