Douglas DC-3 silhouette. The 1952 edition of the Observer's Book of Aircraft used 34 Crown Copyright silhouettes provided by the Controller of H.M. Stationery.  An acknowledgements section on page 5 of the book identifies them by page.  This image is one of those and since the publication date is pre-1957 is in the public domain.
Crown copyright
Observer's Book of Aircraft 1952, William Green and Gerald Pollinger, Fredericke Warne &amp;Co London 1952