It's an old British standard way of showing the switch.
Taking the top left contact set connecting to wire G14 as an example, the upper contact is made when the switch is in the 'surface' position and the lower contact is made when the switch is in the 'nuclear' position. The vertical lines denote the switch positions, so there is one vertical line for each switch position: a six-position switch would have six vertical lines, etc. The black squares denote the position(s) in which the contact is made.