I'm used to seeing three-wire (three phases) overhead mains wiring from
poles along a street. Sometimes you get four-wire (three phases plus
neutral). Modern wiring is a single larger cable which is the three (or
four) wires twisted together. In each case, two wires (between two phases,
or one phase and neutral) go to each house, with a different phase for each
house or group of houses.

But I'm mystified about five-wire mains. As far as I could see, all five
wires were the same thickness and were each fastened to the same type of
insulator on the wooden poles - so probably not three-wire mains and
telephone.

What would the fifth wire be used for?