Colin in the UK sent me these pics of his '31 Model 18; note the left-hand exhaust, Enfield rear hub and Sturmey Archer four speed gearbox.
In addition he sent a few photos (below) of a c1931 four speed positive stop mechanism that he picked up recently. Does anyone recognize the part? Is it a fitment to a vintage Sturmey Archer gearbox? Please let us know, Colin would like to fit it to his Norton. See this blog entry for more info.