From The Strand, July 1908:

“If you ask what Fig. 1 represents, nine people out of ten will tell you that it is a triangular piece of wood fastened to a folding screen on the inside, or something to that effect. It represents in reality a solid rectangular block of wood, with a notch cut in one side. Fig. 2 shows the view looking in the direction of the arrow, the position of the notch being shown by the dotted lines.”