I need your help, what is a "C" tip?

When watching sitcoms from the 1960s, I have heard the phrase "C" tip quite often and I wonder what it means? Is it making reference to a dollar tip or what? I have no idea. Anyone know what a "C" tip is? In one show a person left 2 quarters as a tip and the waitress said there must be a mistake because I guess 2 quarters was a big tip back then. Well, the tipper said you just got half a "C" tip. This is just one example, I have heard it before. Please tell me what you think it means?