The BC class agricultural bank manager's vans were designed to provide a passenger compartment for the bank official, together with accommodation for two horses and space for a horse drawn road vehicle so that the bank manager could visit more remote areas. There were three of these vehicles but by the 1920s the advent of motor vehicles seems to have removed the need for them. In May 1934 BC186 was rebuilt as Z186.

BC186 had retained most of its bodywork to the end, unlike BC185, and so the resulting conversion produced a rather different Z van. Unlike Z185 (later Z20) there was no end platform but there were two sets sliding doors either side of a central guards compartment both of which were equipped with meat hooks for carrying meat. In layout the new van was like a smaller version of the ZJ class passenger vans. As if to confirm this it was given passenger bogies removed from an AZ class sleeper in 1934. It became Z21 in December 1938 and was written off in February 1967.