Collett introduced an improved ‘31XX/51XX’ in 1929. It was pretty much identical to the Churchward design but with modern fittings, outside steampipes and curved frames. This became the standard ‘Large Prairie’ and building continued until 1949, with BR completing the final examples. The first ten filled the gap in the ‘31XX/51XX’ numbering sequence (Nos. 5101‑5110), with subsequent locomotives numbered as Nos. 5150‑5199, and 4100‑4179. The final two, Nos. 4113 and 4161, were withdrawn in November 1965, although No. 4176 lasted as a stationary boiler until 1967. Eight survive in preservation.