Revisiting, I looked at the numbering of the levers and also on the diagram, and thought, “Surely that’s the wrong way round?” Then I looked more closely at the diagram, and realised that it is not the numbering which is the “wrong way round”: it is correct, if the frame faced the other way.
For those who are confused, there is a bar drawn in the signal cabin, which represents not only the frame, but where it is placed. The lever frame in this case would be at the front of the cabin, and the signalman standing behind it, looking out across the station. As such, signals for departing trains would be numbered upwards from 1 in the direction of travel, whereas (presumably down) arriving trains would pass first the distant signal (20) and other signals numbered downwards in the direction of travel.
Were the frame at the rear of the cabin, then the distant signal would be number 1, the home signal 2, etc. On the layout, the control panel is to the front of the station, and the frame is facing the “wrong way”, so the numbering at first sight appears wrong. Given the direction the operator is facing, then the numbering technically is wrong, but the frame numbering is actually as it would appear to the signalman in the cabin.
Neat.