The History of the Korean Martial Arts - Scott Shaw
From link
The evolution of Yun Moon Kwan began in 1931 when, Lee, Kyung Suk, a Korean national who taught Japanese Judo, established the Chosun Yun Moo Kwan school in Seoul. Post World War II, Lee, Kyung Suk asked Chun, Sang Sup to set up a course of Kwon Bop at his school. Kwon Bop is the Korean name for Japanese Karate. This program was named, Chosun Yun Moo Kwan Kwon Bup Bu.
Chun, Sang Sup began his martial arts training in Judo while in high school. He then relocated to Japan to attend, Dong Yang Chuck Sik College. It is during this period that he was exposed to Shotokan Karate and is believed to have earned the Black Belt.
Upon retuning to Korea, Chun, Sang Sup is understood to have secretly taught Shotokan Karate to private students — beginning in approximately 1940. This is how the owner of the Chosun Yun Moo Kwan was aware of Chun. As this practiced was outlawed by the Japanese occupying forces, his teaching was not formally recorded until he established his training method at the end of World War II.
Chun, Sang Sup enlisted the help of Yoon, Byung In to teach at Chosun Yun Moo Kwan. Yoon was a 4th Dan in the Okinawan based system of Shudokan Karate — which was established by Sensei Toyama, Kanken.
Yoon, Byung In taught at the Chosun Yun Moo Kwan for approximately one year before breaking away and forming his own organization, Chang Moo Kwan. Chun, Sang Sup then took over full time teaching responsibilities.