The Merlin engines were used in many aircraft including the P-51, Supermarine Spitfire, Hawker Hurricane and the Avro Lancaster bomber.  This particular engine came out of a racing P-51 that pushed the engine too hard with high performance fuel.  The engine has several large holes in the side of the engine, making one of the pistons visible.  The pilot experienced a loss of power when coming in for a landing.  He was able to land safely.