Ditto in Britain. Our old BS546 (round-pin) plugs were used on both AC and DC systems well into the 1950s.
In DC areas (mostly the older downtown areas of larger towns) and Edison-style 3-wire distribution network was used at anything from 200/400 to 250/500V, with roughly half the houses taking power from one "outer" and half from the other.
Even though the 3-pin (grounding) BS546 plugs were non-reversible, there was no way to ensure correct DC polarity, as the "hot" pin could be either positive or negative of the neutral depending upon which house you were in.
