I've had 3 Gold Stars, a 1955 DB34 Tourer, a 1958 DBD 34 Clubman and a 1951 ZB32 Clubman. All of them had filler caps with butterfly that expands an internal mechanism to fix the cap down.
Attached is a drawing from the 1949-53 parts list showing the type of filler cap. It's part no. is 66-8351