Hmm... these Southern vans were mainly built around the 1928 1929 time period at Lancing when vacuum fitted brakevans were not the norm because most freight trains were unfitted and had no continuous train brake. These vans were early candidates for withdrawal from revenue service and were transferred into departmental uses and coded thus under TOPS, the unfitted vans were coded ZTO... the O meaning without vacuum brake equipment, e.g these were unmodified and mainly remained as built from new at Lancing. The links Matt shows us is for a van coded ZTP which means the van is piped and is not fitted with any control equipment other than a vacuum gauge which is what the external pipe is for, in the same way many unfitted BR standard 20 ton brake vans were modified from unfitted CAO to be redesignated as piped and coded CAP. The later and final modification in demartmental use for these Lancing built vans were to ZTR which means the vacuum pipe versions were given through air piping so that they could run in air braked trains but have no control over the air brakes. Hope this helps.