The twenty-two examples of this class were designed by James Stirling for the GSWR and built at Kilmarnock Locomotive Works (Works Nos. between 95 and 127) between 1873 and 1877. They were numbered irregularly, and not in chronological order between 3 and 115, to fill gaps in the sequence of running numbers. The members of the class were fitted with domeless boilers and a cut away cab. The first locomotive (No. 6) was fitted with a spring balance type safety valve, but the remainder had those of Ramsbottom design over the centre of the boiler.
No. 106 was exhibited at the Darlington Exhibition in 1875.