The cat might have symbolized companionship or represented a certain mood. Maybe it was used to add a touch of cuteness or mystery to the story.
The cat in the 1960s manga could have been significant in various ways. It might have served as a source of comfort for the main characters, or it could have been a visual clue related to some important event. Also, it could have been there to create a sense of nostalgia or to represent a particular aspect of the culture of that time.