The Class 12 was the first locomotive design to originate from the newly established SAR and was acquired to haul coal between Germiston and Witbank.

[3] A number, when added after the letter code, indicates differences between similar tender types, such as function, wheelbase or coal bunker capacity.

[3] At some stage, probably after its engine was withdrawn from service c. 1937, the intermediate draw and buffing gear of Type MP1 tender no.

The modification was done to drawings approved by Chief Mechanical Engineer A.G. Watson in 1929, in respect of Type MP1 tenders of the Classes 12, 12A, 12B, 14, 14A, 14B, 15, 15A, 16, 16A, 16B and 16C.

The new tank increased the water capacity from 4,250 to 4,600 imperial gallons (19,300 to 20,900 litres) and these rebuilt tenders were therefore reclassified to Type MR.[2][3][5]