In 1987 the Korean National Tuberculosis Association (KNTA) seal program used five different images for the 1987 Christmas and TB seals. The sheets are unusual in that there are forty-eight seals depicting mask dances in Korea, and two labels in the middle of the sheets with the Olympics theme for the 1988 summer games, which were held in Seoul. In each sheet there are ten of the first three seals and nine of the remaining two seals. Also issued was a souvenir sheet of ten seals with two sets of the five images.
The 2022 Korea Postage Stamp Catalogue (KPSC) gives the catalogue numbers for the 1987 seals as TCS 223-227 for the five different images. The KPSC values the set of five seals at 3,000 won (US$2.25). The KPSC catalogue number for the souvenir sheet of ten is TSC 227a and it is valued at 6,000 won (US$4.50). The KPSC did not give catalogue numbers for the two Olympic labels, nor any value.
What a shame. I am a Korean, but I did not know about this bizarre seal item with Olympic mascot and logo.