In 1984 in Seoul, I managed to pick up two NHK domestic outlets on shortwave. NHK Kumamoto 6130 kHz (JKQ21) sent nice handwritten letter for a report of their transmission. NHK in Tokyo responded with a Radio Japan QSL card for a report of NHK Hiroshima 6175 kHz (JKG21). They put out fair signals into Seoul and were quite readable.

I am not sure but I believe these were links from the key stations to their subordinate stations.