A gas-liquid rotating coalescer includes first and second sets of one or more detent surfaces on a rotary drive member and a driven annular rotating coalescing filter element which engagingly interact in interlocking mating keyed relation to effect rotation of the coalescing filter element by the rotary drive member. Designated operation of the coalescer requires that the coalescing filter element include the second set of detent surfaces. A coalescing filter element missing the second set of detent surfaces will not effect the noted designated operation.