Grid reporting system used. Based on North/South and East/West lines centred on the Isle of Wight, the grid comprised 100 Km squares each given a two letter code, in this example 'uP'. Each 100 Km square was then subdivided into 10 Km squares and subsequently into 1 Km squares, with numbers used as per the diagram below. Thus and aircraft position could be referenced to a 1 Km area by a two letter/four number code, e.g. 'uP2636'. 
(From AP1093C)
 1940s ATC Pages
An Island Images webpage © Jon Wornham