Update on the postwar filters. The attached sketches are from the 1954 handbook for the 200 gallon water tank. The filters are clearly quite different (and larger) than the picture Guy posted. For an idea of size, there is a picture of a postwar filter installed on a 100 gallon water trailer here (2nd posting, 3rd picture):
http://hmvf.co.uk/topic/14759-britis...tank-trailers/
So, my working theory is that the one in Guy's picture is 'late WW2'. At least until someone can confirm otherwise.
Owen.