This applet draws a 4-stage (4-bit) full adder circuit which uses only 6-pole 2-way switches and indicators. The indicators shown are pairs of LEDs in green and red, together with load resistors. Torch bulbs would be equally viable but their consumption tends to rule them out.
The circuit uses coloured lines to indicate positive and negative potentials and these change when the switch positions are varied. The switches in the circuit are operated via the buttons at the top.
The two rows of buttons represent the numbers to be encoded on the switches. The state of these buttons is indicated on the top two rows of indicators. Their sum is shown on the bottom row of indicators.
Part of the circuit may be "re-wired" by pressing the add button. The circuit re-arrangement affects the generation of Carry and not-Carry signals such that it then performs subtraction.
I have built an 8-stage working version of this circuit. (Pictures here.)
The add/subtract switching was achieved by a further row of switches but the additional circuitry for this change of function complicates the diagram unnecessarily.
[If you want to play with the display, the Java source is available at: source zip file.]