Lets parse this one. The unit, "X Super Heavy Battery RCA" did exist. It was operational for a grand total of about 4 months, from October of 1940 to February of 1941. The personnel were drawn from the CAHU (Canadian Artillery Holding Unit)to form "X" and "Y" Heavy Batteries, operating railway guns in defense against a possible German invasion. During their period of operational control they were under command of 3rd Super Heavy Regiment RA. But they were only a unit for four months.
To come at it from another angle, the D xxxx number, either a D6xxx or an 8xxx were blocks alloted to artillery units, so that supports the artillery connection.
Unfortunately, the formation patch has been obliterated, and is not really discernable. But, if there was as formation patch worn by 3 SHR RA during the time period it needs to be checked out.
Balance of probabilities? Why would a gunner leave the identification of his helmet to that unit? It would be incorrect after Feb 1941.