The months of July and August 1950 were very difficult months for Seoul. The North had taken over the city very early on in the Korean War and the situation was, to put it mildly “hard”. Atrocities were being committed on a large scale and the ordinary life of the city had basically come to a standstill. This however did not stop all sorts of administrators doing what they thought was their job, including the (Northern) postal service employees in the city who organised the overprinting of South Korean definitives. Many examples of these overprints are philatelically-speaking “suspect”, with huge numbers of fakes created.
Of course, we do know for a fact that such overprints were actually used; they are not all fake.