Some of the later Viscounts had the ground power socket behind the wing on the left side of the fuselage. SOAF 501 and 502 were like that.
The batteries and power distribution was in the belly forward of the wing spar (G,H,J&K panels-sad that I still know). Most A/C had the socket near this equipment to save cable weight I suppose.
The 4 Voltage regulators were under the cabin floor at this location and setting them up so that they all shared the load equally could take an hour or more doing ground runs, with much shouting from the cockpit to adjust one or another regulator. Just when you thought you had got them all balanced, tightening the lock nuts would upset them all again.