I'm not sure, but I believe they're supposed to look like cork - probably a component in early cigarette filters which were more of a specialty item, than a regular part of the manufacturing process before the 50's.
It is supposed to look like cork, but i think they must've have studied it or something, it must have some psychological effect on the consumer. Faux Increased Quality perhaps?
Even though the question was answered for the most part- one thing was left out: http://en.wikipedia.org/wiki/Cigarette_filter#History