Nice explanation. Here in the u.s.a., this switching configuration is known as the carter 3 way, also sometimes called farmer's 3 way among other nicknames and has been prohibited by the National Electrical Code since 1923, but has continued to be illegally installed for decades afterwards. Most often seen with knob and tube wiring systems but also common on farms and in rural areas, where you have an outbuilding like a garage or barn, a 3 way switch in the house and outbuilding, controlling a yardlight in the middle and receptacles in the outbuilding constant hot, using only 3 wires instead of 4 (excluding a ground) This configuration can also be on two different circuits, so if you were doing work in the service panel and got the phasing mixed up you would send 240 volts to the bulb in one of the positions that would turn the bulb OFF (Hot-Hot) in the olden days during the depression and wars, people were very fugeral and if they could find a way to save a dollar here and there, they would do so.
