They became famous, in Britain, for their illustrations to the story by Diana Ross of The Little Red Engine. The first story, The Little Red Engine Gets a Name, was published in 1942. The book was published by Faber and Faber.
Here are some pictures of the illustrations. There is no note about the printing. But, the colour litho is different in quality from the book featured previously. My guess is that these books were printed using plastic-sheet colour separations of photo-mechanical offset litho. Not the same; but lovely nevertheless.