 The U1 chimneys replaced the standard N class type on the earlier locomotives, which, along with the removal of the piston tail rods on the earlier batches, created a truly standardised appearance.In 1937, Maunsell's replacement Oliver Bulleid saw no need to improve draughting of the class, and spared them from trials with Lemaître multiple-jet blastpipes and wide-diameter chimneys.However, he had Maunsell's smokebox-mounted anti-vacuum snifting valves removed at the end of the Second World War in an effort to reduce maintenance.