1 Sen - Shōwa

By 1943, Japan's wartime aluminum allocation had become so constrained that the government was simultaneously reducing coin weights and requisitioning existing metalware from civilian households. This 1-sen piece was struck at just 0.55g — roughly half the weight of its prewar aluminum predecessors — as the military supply chain consumed priority stocks. Production was discontinued entirely the following year, making the 1943 issue the final year of the denomination in circulation.