Most collectors of South Korean stamps are familiar with the 1946 set of Japanese stamps overprinted for use in US-occupied South Korea and revalued in Korean currency, i.e. cheon. These are listed in the Korean Postage Stamp Catalogue (KPSC) as numbers R1-R6 and in Scott as numbers 55-60. They were issued some months after the Liberation (replacing unoverprinted Japanese stamps) and used until the introduction of South Korea’s first definitives.
During the same period, 2, 3 and 5 sen Japanese Nanko postcards were also surcharged locally for use in South Korea, with the 5 sen being the most common. See Chen Fi-Yu’s article of February 3, 2021, published on the Korea Stamp Society’s website for the background, and more images of these Japanese Nanko postcards . These Nanko postcards were also surcharged for use in North Korea until mid-1948.