Here is a new reproduction fuel filler box, correct for a Jaguar XK120, XK140, or early XK150. The hinge bolts on at the front, so it is for front-opening filler doors only. The late XK150 used a rear-opening fuel filler door and a different type of filler box.
There is some light surface rust on this part, but that will xclean off easily with some steel wool or light sandblasting, always a good idea with new steel parts anyway to ensure a good clean surface for paint adhesion. It was made in England by a major maker of new reproduction parts for vintage Jaguars.