Korean yen
The Korean yen was the currency of Korea between 1910 and 1945. It was equivalent to the Japanese yen and consisted of Japanese currency and banknotes issued specifically for Korea. The yen was subdivided into 100 sen. It replaced the Korean won at par and was replaced by the South Korean won and the North Korean won at par.
North Korea's illicit activities
The International Convention for the Suppression of Counterfeiting Currency is the primary treaty whereby states agree to criminalize acts of currency counterfeiting. North Korea is not a party to the treaty.