This is a 2 Bit Relay operated Subtraction Machine. 4 SPDT switches used for two input numbers. After processing the numbers through Relays answer showing on third and fourth display. Wait for 10 to 15 seconds to stabilize all Relays to get proper Answer. The full circuit and actual model can be watched in my YouTube channel "Basic Knowledge."
https://youtu.be/UP8_W4ZRVUs
