It was all down to the dimples....
More on that later. When it came to my listening of Seiko Matsuda's（松田聖子）discography, I've always treated "Aoi Sangousho"（青い珊瑚礁）, which was her 2nd single, as the beginning. It was after all the breakthrough song for her when it came out in July 1980, and it was also one of the first songs that I ever heard by her via "Sounds of Japan" on the radio.
However, as I just said, "Aoi Sangousho" was the second single. Therefore, there was the debut single, "Hadashi no Kisetsu" (Barefoot Season) that had come out about three months before in April. It is a song that I have heard before by Seiko but hadn't realized until just a few years ago that it was Single No. 1 for The Eternal Aidoru.
Much obliged, J-Wiki.