In 1942, Edgar Alcock, the Hunslet Chairman, convinced the Ministry of Supply that the Hunslet Class 50550 18” Saddle Tank, being of a more straight-forward design than the Ministry favoured LMS Jinty, would be more suitable locomotive to meet the Ministry's robust demands for military operated locomotives within the UK and overseas.The first locomotive was completed at Hunslett's Leeds works at the start of 1943, with many being subcontracted to Andrew Barclay Sons & Co., W.G. Bagnall, Hudswell Clarke, Robert Stephenson and Hawthorns and the Vulcan Foundry, in order to meet delivery requirements. In all, 347 had been built for the War Department by 1947, with two further engines being built, unauthorised, for colleries.A further 77 new Hunslett Austerities were built for the National Coal Board and in 1956 one loco, 3846, was allocated to the Graig Merthyr Colliery. Being required urgently, locomotive 3844 was more complete than 3846 and so Hunslet simply changed the plates to allow 3846 to be delivered instead. The swapped 3846, with no NCB identity, worked until June 1978 when the colliery closed. Eventually rescued in 1997, when she was bought for use at the Appleby-Frodingham Railway Preservation Society, the error was discovered and the 3844 Works Plate was restored to the loco by the owner, Glenn Britcliffe.