Hoei Yotsuho Cho-gin, Japan, 1711 - an image of the god of wealth and the Chinese character takara, "treasure", are stamped numerous times over the face. (Image Curtesy of the Bank of Japan's web page)
Japanese silver money was measured in weight by the momme of 3.75 g. The regular fineness of silver minted in the 17th century had been 80%, the devealuation of the coinage of the 1690s brought - to increase the amount of coins circulating in the isolated nation - the fineness down to 50 percent. From 1706 to 1710 the coinage was diluted in four further steps down to the poorest-quality cho-gin, with a silver content of merely 20 percent, the Hoei Yotsuho Cho-gin. To satisfy Korean traders who refused to accept Japan's coins for their face value Oko-gin - silver coins with a fineness of 80% - were minted from 1710 to 1714 for the exclusive purpose of being used abroad.
See also: Money (Japan)
Bank of Japan (ed.) Short Essays on Monetary History Contained in Monetary and Economic Studies, 1-9 Oko-gin Silver Coins for Import of Korean Ginseng: A Two-Tiered Pricing System for Silver Coins link
Subpage of the Marteau Platform of Research in Economic History