There was probably a difference between the densely populated cities and metropolitan areas, like London, Liverpool, Manchester, Birmingham, Glasgow. Which later on became 01; 021; 051; 061 etc.
Some rural sparsely populated areas still had three digit numbers in the 1970s.
Dials used to have letters as well as numbers, just as the buttons on mobile phones.
Each exchange in the major cities had a name like Whitehall; Central; Bootle, and the first three letters would be used.