Heusinger-Steuerung
When the need for more branch line locomotives became apparent in 1934, tenders were invited by the South African Railways (SAR) for another fifty Class 19B locomotives with Walschaerts valve gear, but redesigned by Chief Mechanical Engineer A.G. Watson with his Watson Standard no. 1A boiler. When the tenders were received, it was found that the North British Locomotive Company of Glasgow, Scotland, had also tendered for a locomotive with rotary cam poppet valve gear as an alternative to Walschaerts valve gear.