Probably 1957.

Original numbers 3111-49, were 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,

3150-90, very large prairies, No.4 boiler
3100 (the second)-3104: were 3150s fitted with smaller wheels for extra power.