This was the second batch of standard Southern Railway shunters based on the English Electric 6KT 350 hp (260 kW) diesel engine. These locomotives (BR numbers 15201-15203) were built by the SR in 1937 and were later classified D3/12.
The Class 12 locomotives were built at the BR Ashford Works over the period 1949–1952. They were numbered 15211–15236 and became Class 12. No locomotives survived long enough to acquire Class 12 TOPS numbers though.