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 p.s.i. and using smaller coupled wheels of 5 feet 3 inches diameter, and a half inch increase in cylinder diameter, nominal tractive effort rose to 31,170 pounds. Almost impossible to pick out was a 2" reduction in pony truck wheel diameter to 3'. 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.
Whitehurst, Brian (1973). Great Western Engines, Names, Numbers, Types and Classes (1940 to Preservation). Oxford, UK: Oxford Publishing Company. pp. 30, 102, 135. ISBN978-0-9028-8821-0. OCLC815661.