These coaches were converted from BSKs, and were modified internally to provide toilet and seating accommodation for accompanying security staff. Their function was to transport bullion and bank notes. They had air brake equipment added, and so were dual braked. The end gangways were removed to enhance security, and the glazing was heavy duty double glazed units that appear to be either tinted or mirrored in available photographs. The toilet windows appear to have been fitted with opaque white glass. The roof ‘pods’ contained radio equipment. B5 bogies were fitted upon rebuilding.