Opposite rotating props. Not with an idler gear, the whole engine rotated in the opposite direction. Rolls-Royce never did that again – it seems it wasn’t one of their better ideas.

The 130 series merlins used in the DH Hornet used an idler gear to reverse the direction of the propellor rotation. Both engine crank shafts rotated the same direction. One engine installation was actually longer on one side than on the other accordingly!