This was back in the days of the infamous Heath government who had a major reorganisation of county boundaries in 1974. This created Metropolitan County Councils such as Tyne and Wear, Greater manchester and so on. These were abolished by Thatcher in 1986 but the names still remain as postal addresses. Until then everything down to the north bank of the Tyne was Northumberland and everything south was Co. Durham. If you leave Newcastle by the main east coast line, half-way across the bridge is still the old sign pointing to Northumberland and Durham.