This one?
ETA: That seems to be a dead-on match for this:
which is called
migi futatsudomoe. I can't seem to find a reliable source indicating exactly what this symbol means, although some web sites suggest that it has a meaning in Buddhism similar to that of the more commonly known "yin and yang" symbol.
This makes sense, given that
zazen means "Zen meditation". Plus, it also kind of looks like a stylized letter S. Like many good logos, it's a visual pun.
