Two years later, four more of this type of boiler were ordered when No.87 was constructed out of Class A No.63 and Nos.72 and 73 were rebuilt from Class U to U2. The remaining boiler was presumably to have a been a spare one to enable speed shopping of engines but in the event it was immediately fitted to No.78. This engine was the last of the NBL engines to be built in 1924 but she had run a considerably greater mileage () than other members of the class by the time she received the new boiler in October 1936.