Complex sentence: They were identical to predecessors Class 16 and Class 16B in most respects, except that Hendrie had added a combustion chamber to the boiler, similar to that of the Class 15A, externally visible as an extension of the Belpaire firebox hump.
Split into simpler sentences: They were identical to predecessors Class 16 and Class 16B in most respects, except that Hendrie had added a combustion chamber to the boiler, similar to that of the Class 15A. The presence of the combustion chamber was visible externally as an extension of the Belpaire firebox hump.