These locomotives were a reinforced version of the standard goods locos that James Manson had had built at Kilmarnock a few years earlier. They had larger cylinders and a larger wheel diameter. Modern features were a steam-powered reverser and steam-powered sanders. The first 20 were built by Neilson, Reid & Co., twelve more in 1907 by North British and a further two in 1910 by G&SWR's own workshops at Kilmarnock.
Sectional drawing as built
Locomotive Magazine, September 1921
In 1920 Robert Whitelegg had these locomotives rebuilt with new boilers. Since he wanted to standardize the boiler types, these were the same as some on newly rebuilt passenger locomotives with a 4-4-0 wheel arrangement. They received a new driver's cab, but no superheater.