After World War One in 1918, more 20-ton vans to the improved design were built with G W R self-contained buffers and given Diagram AA15. A total of 378 were built between 1918 and 1927. The next development was the AA19 which were built between 1927 and 1931 incorporating many RCH parts such as buffers and axleboxes but utilising the same body. The total constructed was 221 vans. Diagram AA 20 followed which was externally identical to the AA19 vans with a total of 354 being built between 1934 and 1943.

The Diagram AA21 was for 100 Vacuum fitted vans built 1939/1940, being built with full vacuum brake gear with the brake cylinder in the front of the brake veranda.