The Black Trademark (1963)

Ide of the Kokusai Rayon Company visits a supermarket and finds cheap shirts being passed off as his company's products. But a closer look reveals that the last letter of "Kokusai" has been changed to "n" and reads "Kokusan." The buyers, however, are all under the impression that they are buying Kokusai shirts. Ide begins an investigation to find out where the shirts are being made, but before he is able to uncover anything, he is killed on a train and his little black book, filled with secret data, is stolen. Sugino then starts a one-man search for both the killer and the factory turning out the cheap shirts.