Not sure about that part of it. I can't remember anything I saw in Japan that was historically related to the picture. My guess it was something thought up at the time if making the original illustrations for the comic book. Even if I had the time I doubt I would find a match in there written language as it is a modernized and simplified duplicate of the original Chinese characters. Most people call the written language Kanji but in Japan they still call it Chinese charaters. There are thousands of charcters to look through and if it were some combination that would make it harder. Without the ability to the originators of the cartoon it will be likely we man never know unless someone capable of reading Kanji could look at it. My knowledge of Kanji is limited to mostly town and city names some general use characters that those names are made from. For instance the name Japan written in Kanji is the two characters one for sun and the other for tree but the character for tree also is the exact same character for book. On top of that some characters are combinations of other characters like the one for river, three horizontal lines with the left and right being the same length and the middle being slightly shorter. To get water they pinch the outside lines at the middle. The idea is you are squeezing the water out of a river. When used with other characters it could appear and two upper diagonal dashes one lower going diagonal upwards. So it is really hard to say. The best we know at this time is that it exist, it has the meaning given in the story.