Many thanks, Martin, David and Dave. All photographs are different, so it is a bit confusing. However, I am leaning towards the lining as it is shown on page 26 of the 1912 Scott catalogue for 1912. It is impossible to see from the catalogue picture whether top and bottom of the tank is painted in a different colour (black). With the orthochromatic film used in those days, black and purple would look identical unless filters were used. Hopefully somebody knows for certain. In the meantime I will lean towards Stan Greenway’s interpretation with purple painted tank from top to bottom.
More information will be received with thanks.