A3s 2747 "Coronach" and 2751 "Humorist" were subjected to smoke deflection trials following an accident on the London, Midland and Scottish Railway (LMS) due to poor visibility; this included the modification of the upper smokebox area surrounding the chimney. Originally the whole smokebox wrapper was retained in order to form an air duct, with the exit behind the chimney, but this was found ineffective. The next stage, at least with 2751, was to cut off the top part of the wrapper, but retaining the sloping plate that directed air flow upwards, and therefore lifting the smoke above the locomotive. The original chimney was replaced by a double stove-pipe variety, and miniature deflector plates were added on either side, angled to concentrate the air flow when the locomotive was on the move.