1 Mon 'Kan'eitsūhō' Copper alloy, 小 'Ko' reverse

The Kan'ei Tsūhō series had circulated since 1626, but the 1737 reissue — designated by the small 小 (ko) mark on the reverse — was a deliberate quality reduction. The Shogunate debased the copper content relative to earlier issues, funding Tokugawa fiscal obligations at the expense of coin integrity. Merchants and money changers recognized the difference immediately, and exchange rate friction between old and new Kan'ei pieces became a persistent irritant in Edo-period commerce.