The extension of the Mark X unit bodies was done by Motor Panels, a subsidiary of Rubery Owen. The floor pan with mechanicals was available to coachbuilders as a rolling chassis for use with specialised bodywork, usually as hearses; Startin of Birmingham built more than 300 DS420-based hearses. Finishing from the bare metal, including final assembly and trimming the interior, was done by Vanden Plas, who had earlier made the Princess for BMC.[152]