Has anyone considered the possibility that it could be an earth wire (TNS earthing system).
I have seen this arrangement on a small number of rural supplies where there is only a handfull of consumers.
I saw it recently when doing an upgrade, the incoming PoCo supply was split-concentric cable, with separate neutral & earth, not jointed at the cut-out. I traced the cable to the pole outside, there were L,N & E wires on the pole & I could see the house service terminated to these.
The lines on the pole were fairly old (1950's) but the drops to houses had been renewed with split-concentric cables.
I think the ground in the area is very rocky, so achieving the impedances for PME may have been difficult.
The PoCo here used to call these supplies C.E.W. (Continuous earth wire).
