The SR West Country and Battle of Britain classes, collectively known as Light Pacifics or informally as Spam Cans, are air-smoothed 4-6-2 Pacific steam locomotives designed for the Southern Railway by its Chief Mechanical Engineer Oliver Bulleid. Incorporating a number of new developments in British steam locomotive technology, they were amongst the first British designs to use welding in the construction process, and to use steel fireboxes, which meant that components could be more easily constructed under wartime austerity and post-war economy

Due to the problems experienced with the class, and following the success of the rebuilt Merchant Navy class designed by R. G. Jarvis, British Railways ordered the rebuilding of sixty locomotives to a more conventional design at Eastleigh between 1957 and 1961.^[54] The first locomotive to be rebuilt was No. 34005 Barnstaple, which adopted many features from the BR 'Standard' locomotive classes.^[55] The casing was removed and replaced with conventional boiler cladding, boiler pressure reduced to 250 psi (1.72 MPa) and the chain-driven valve gear was replaced with modified Walschaerts valve gear fitted both outside as well as between the frames.^[18] The rapid onset of the 1955 Modernisation Plan during the early 1960s meant that the remaining fifty locomotives were not rebuilt, and continued in as-built condition until withdrawal.