Immediately recognisable by its water tanks, which gave rise to the generic description of Pannier tank, engines commencing with 8750 were all built with improved cabs for crew visibility and weather protection.
Other detail differences were included, such as the prominent shield behind the whistles, to deflect steam, and quick release oval tank fillers.
For purposes of identification, the pannier tanks were subdivided into three groups numbered 5700, 8750, and 9700, the latter having condensing equipment - the numbering sequence became considerably more complex than this.
The class was designated the Power Classification of 4F in BR days, and was originally selected as the standard GWR shunting and general purpose tank enginer from 1929 onwards, at first carrying a blue R.A. spot until 1950, when it changed to yellow. The standard BR livery of unlined black was applied, but one or two engines carried full mixed traffic lined livery for Paddington station pilot duties.
3738, preserved at Didcot Railway Centre, was the first of the type to be rescued from Barry scrapyard, and restored to working order.