The story
The first Japanese post office in Korea was established in 1876 under the terms of the Japan–Korea Treaty of 1876 in Japanese consulate district within the open port of Busan. Post offices in Wonsan and Incheon (Chemulpo), ports which had also been opened to foreign trade by the treaty, were established in 1880 and 1883, respectively. A post office was also established in the capital of Seoul in 1888. Additional Japanese post offices/agencies were opened from 1896, particular from 1899, first in other treaty ports including Masan and Gunsan, then other inland places, predominantly as a political measure, as few initially made a profit.
Initially, these post offices used stamps of Japan; however, from January 1, 1900 through April 28, 1900, stamps of Japan were overprinted with the kanji for Korea (朝鮮, Chosen).
From Wikipedia’s article on Japanese post in Korea, shared under CC BY-SA 4.0.3