Not collected in the Ishinomori Complete Works.

(from ebookjapan):
Hero comic master Shotaro Ishinomori emotionally depicts the subtleties of various people's hearts in this human drama. Hiroshi sees a message on a postcard from his ex-girlfriend Hiromi, who married another man: "Are you still using a fountain pen? Hiroshi remembers the time he received a fountain pen from his ex-lover Hiromi, who married another man. Then, as his memory goes back, Hiroshi recalls a sweet and sour event with Hiromi in high school ......!