The first batch of twenty locomotives was ordered in April 1941, although the changes in design to the "Light Pacific" arrangement meant that production was delayed until late 1944. Due to wartime contract work at Brighton works, the boilers were built under contract at the North British Locomotive Company. Before the first of the class had been delivered, the order was increased to thirty, with a second batch of ten ordered in September 1944. Deliveries from Brighton works began in May 1945 with prototype No. 21C101 "Exeter", and proceeded at the rate of about two locomotives per month. The class was gradually run in on the Central Section until October 1945, when they were successfully trialled on Plymouth and Kentish services. By the time the first fifteen had entered traffic a further order of fifteen was placed, with these entering service between June and October 1946. From this batch onwards, traction was improved by the addition of steam sanding to the front driving wheel, with covers added to protect the motion from sand falling from the filler pipes.