Article by Arne Barker, Area Representative Glos Wilts & Bristol, 2010. Text from 1963.
Formerly, the letters came before the numbers, latterly the positions have been reversed to provide further combinations. These are issued by a County Council, County Borough or Burgh Council (designated below – C.C., C.B.C. or B.C., respectively). Each authority has a set of one or two code letters and these are listed below. In tracing the origin of a plate with three letters the last two will indicate the issuing authority, e.g. a car bearing a plate ARA or ARB will have been registered in Derbyshire, as RA and RB are the code letters of that County Council. An exception to this is GPO. This is an index issued by the London County Council to the General Post Office.
Click on the second letter of the three letter combination in your registration – eg for ARA click on R then look up RA