I found these items in a box of fittings I purchased from the Murray Bridge area of South Australia. The area had a 250 VDC supply prior to the 240 VAC 50Hz being standard in the area.
The three pictures show the plug & socket. Unfortunately, the socket has a bit missing around the Earth pin. The markings on the back of the socket are:
MADE IN ENGLAND VITREOUS 14522 5A 250V
The manufacturers' mark looks like a horseshoe magnet with wire around it with the letters GEC with this mark.