 While almost identical externally, they had their boiler pressure increased from .With the introduction of the 17th class (numbered 271–280) and the 18th class (numbered 281–289), the early members of the class were renumbered to put all of the 15th class into a solid block.Over the following years there were some swapping of boilers between locomotives of 15th, 15A and 16th classes, as well as exchanges of front bunkers, so it was impossible to say for certain if a particular locomotive is 15th or 15A class (i.e.