Evidence for how Japan made naval armour is much thinner than the surviving data on its performance. The workers involved left no usable accounts, and the relevant industrial records did not survive. Deliberate destruction at the end of the war preserved secrecy but removed the evidence needed to reconstruct production methods and material mixtures. Surviving armour samples and tests therefore cannot answer every question about the industrial system behind them.
Notable Point · 2:35:17-2:37:28
Why Japanese naval armour production remains obscure
The destruction of Japanese industrial records leaves historians unable to reconstruct important details of wartime naval armour manufacture.