The black quadrant above is not a Mark 4 one as it has too many teeth,ans is half moon in shape.A Mark 4 quadrant has less teeth and is more like a quater circle in shape,as SafeChav siad,thats where the name came from.

click ere for a picture of a Mark 4 quadrant,hold you're curser over the picture of the white Mark 5 picture to see it.

http://www.apracing.com/roadcar/clutch/epmods.htm

Regards,Micky