Now a guess on my part. Since the formation in 1940 had 3 Regts and an OCTU, could this be represented by four sides of a square?
This from Richard Doherty's "Ubique. The History of the Royal Artillery in the Second World War"
coastartillerytrainingcentre 1940formn.JPG
Charlie
edit. There is a WesternGazette newspaper clipping ( in my copy of History of Coast Artillery) dated 1957 which mentions the closing down ceremony at the Citadel and the recent closure of CATC. So I am Now assuming that CATC HQ remained at Plymouth 'till the end although Depot established at Dover? I presume they all wore the same formation badge?