This class of large prairie was created in 1938 when Collett rebuilt some of Churchward's 3150 Class with a view to using them as bankers, particularly from Severn Tunnel Junction shed. These engines used the standard class 4 boiler again, but pressed to 225 psi (1,550 kPa). and using smaller coupled wheels of 5 feet 3 inches (1.600 m) diameter, and one-half inch (13 mm) increase in cylinder diameter, nominal tractive effort rose to 31,170 pounds (14,140 kg). Almost impossible to pick out was a 2 in (50.8 mm) reduction in pony truck wheel diameter to 3 ft 0 in (914 mm). Only five engines were ever modified, namely 3173, 3156, 3181, 3155 and 3179 which were rebuilt as 3100 to 3104 respectively. None of these were preserved.