The sanitation engineer must have had a great time deal of fun pulling together all of those various parts and then being told that the design had to go onto the rear of a lorry. This is a much overlooked element of a division's support services. Many personal war diaries tell of visiting the mobile bath and laundry units when rotating through in reserve and the delight of getting thoroughly cleaned. That and local cafes, ENSA shows and getting fed a proper meal.
Does anyone know what water supply was used to keep the steam flowing? I suspect that somewhere on the rear of the lorry is a pump and filter to use natural water sources or Royal Engineer constructed water tanks but could a water bowser be connected?
The Imperial War Museum(IWM), in London, have a model of a 'Mobile Disinfector Truck' with a description "Scratch-built model of a Second World War period British/Indian Army disinfector truck, having a wooden chassis and wheels, metal superstructure (bodywork), and glass plate windscreen. The open flat-backed rear of the truck carries a cylindrical metal boiler and stands for two disinfector drums. The drums themselves are not present. The model (based on a standard 3-ton truck) is finished in a black and brown paint camouflage scheme and is marked with the following painted (in what was originally white paint) distinguishing numbers: '25TH / F.H.S. / 602' (on left front mudguard); 'L 14794' (on left and right side of bonnet); 'W / broad arrow / D' (at rear end of flat back)."Height 126mm, Length 270 m, Width 120mm", Catalogue Number: MOD 2494: https://www.iwm.org.uk/collections/item/object/30115600 (sorry - no picture). No other information jumps out of the IWM collection. Maybe more research will reveal something.
I was hoping to identify the 3 ton lorry manufacturer but can't make sense of the WD Number 'L14794' from the official lists.
Perhaps '25th FHS' is 25th Field Hygiene Section? The FHSs were part of the Royal Army Medical Corps (RAMC). The National Archives in Kew has War Diaries for the British and Indian 25th FHSs: http://discovery.nationalarchives.gov.uk/results/r?_q=25+field+hygiene+section