Super interesting read; I especially found this letter from "P791" fascinating:

> Hi, Wonyoung, I map North Korea to help North Koreans.I select areas that look like they might be good points to escape. In particular, the detailed mapping around Phungso-ri (풍서리) is almost entirely my work.I mostly map along the Tumen River, both on the North Korean and Chinese sides. It is important to map the fences, roads, and towns on the Chinese side.