I have not seen it associated with any motorcycles, but I have seen it associated more with para bicycles... its the bottom one in this image.
But if it is '39 dated I dont think that could be associated with the para bike as it would be too early for that ?
Richard. https://picclick.co.uk/2X-Ww2-Era-Vintage-Bsa-Para-Bike-Spanner-123085661791.html