Stanier developed these locos from Fowler's 2-6-2T. The biggest change was in the new boiler, which was tapered towards the front. The locomotives with the numbers 71 to 120 and 121 to 144 each used a slightly different variant of boiler type No. 6 without a dome. Numbers 145 to 209 had boiler no. 6A with a feed and a steam dome. In general, the locomotives are referred to as Stanier's least successful design. Among other things, the extremely small superheater surface is mentioned. Six of them were later given the 6B boiler, but this was not cost-effective. Nevertheless, they were only retired between 1959 and 1962. 