With GW style BR lettering. A Collett engine that has (inexplicably) been fitted with small-boss Churchward-era driving wheels.

Paul Hamley explains :
3100 (the first) ex prototype No.99, No.2 boiler. Later became 5100 and then 8100
3101-10 small prairies became 4401-10 in 1912 renumbering, class prototype 115 becoming 4400
3111-49, large prairies, No.2 boiler, inside steampipes, thin boiler support in front of tanks, small wheel bosses, straight drop frames, red restriction (?). Rebalanced by Collett to attain blue restriction and renumbered 5111-49. Some received outside cylinders and curved frames, see 3128 on this site
3150-90, very large prairies, No.4 boiler
3100 (the second)-3104: 3150s fitted with smaller wheels for extra power.