A 1937 B.S.A. catalogue in my collection illustrates a similar gun described as their," Ejector Model De Lux ",priced at 22 pounds, as compared to their standard model at 18 pounds,10 shillings.
The other possibility is that based on the action profile that it is a version of their wildfowl ejector gun that sold for 20 pounds.
B.S.A also sold a Special game model that sold for 25 pounds, these guns have profuse rose and scroll engraving.

Roy Hebbes