I remember those, Had to drive to three different Autozones to get the plugs for my 85 Rx-7. Gotta love those rotary engines. The reason that they are shaped that way is to allow the apex seal to pass over the end of the spark plug without hitting an electrode (in this case the four quarter circles surrounding the center) but still allow ample room to spark the mixture. There are also two different spark events per ignition sequence, called leading and trailing. They help keep the rotor moving in the right direction.
