Brian,I`ve looked through my books and can`t find anything definitive,except I read ,perhaps in an American mag. that the rotation was changed,(swop engines across) because of vibration,or slipstream interference on the tail,which may have been because of the twin-booms.I don`t think it was fully cleared until the `J`and`L` models.Various other mods were made,including `spur` gearing,raising of the thrust-line,,intakes reshaped,change of props to Curtiss Electric,raising tailplane incidence,and electric airbrakes,and `beard` radiators..reference for that is `Famous fighters of WW2`,by William Green.