The Douglas factory stamped some parts, like this tappet from a 1936 Aero, with the DK symbol shown in the picture. DK stood for Douglas Kingswood. Their stamping ‘policy’ is not entirely clear, as the other three tappets are plain – but next time you’re rooting through a box of bits at an autojumble and come across the DK symbol, at least you cannot say you Don’t Know – it’s definitely Douglas! Did this stamping practice continue post-war? If you now the answer, please add a comment below.