John Weller designed an advanced car powered by a 20hp 4-cylinder engine which, with the gearbox was mounted on a detachable underframe which was separate from the chassis. Final drive was by double chains. When the clutch was fully depressed reverse gear was automoatically engaged, providing an emergency brake. The Weller would have been expensive to make commercially, so its backer, butcher John Portwine, suggested a much cheaper machine which appeared as the Autocarrier, a trade carrier whose passenger version was the A.C. Sociable.
Source: Nick Georgano / The Beaulieu Encyclopaedia of the Automobile