Complex sentence: The system of grouping narrow gauge locomotives into classes was only adopted by the SAR somewhere between 1928 and 1930 and at that point these six locomotives as well as the three Class Hd locomotives were all classified as Class NG5.
Split into simpler sentences: The system of grouping narrow gauge locomotives into classes was only adopted by the SAR at some date between 1928 and 1930. At that point, these six locomotives and the three Class Hd locomotives were all designated Class NG5.