It is a Korean Sangpyeongtongbo dangbaekjon (상평통보 당백전) cash coin.
To add, the dangbaekjon that you posted is a cash coin that was modified after being cast. See, these were supposed to be round in shape, but some of these cash coins were repurposed/ modified to be used as charms or amulets. Some of these went even further than just being made into an octagonal shape - some were attached to a chain, enameled, etc. They are called gagongjon (가공전) in Korean.
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