Complex sentence: Although the class was augmented by a further 60 engines between 1900 and 1909 with another ten added in 1917-18, the final five built reverted entirely to the original 1889 specification.
Split into simpler sentences: Although the class was augmented by a further 60 engines between 1900 and 1909 with another ten added in 1917-18. The final five built reverted entirely to the original 1889 specification.