I'm sure that I had a backplate made from a 1/16th inch thick bakelite, just held in place by the mounting screws. I do also have a memory of similar backplates held on by eyelets around the screw holes but I'm not sure what they were on.
But if you don't have a cover then it fills up with salt if you ride in the winter..JohnWith a well- sealed backplate you are perhaps providing a better micro-climate for corrosion should any moisture travel along the operating arm impelled by your rapid forward progress. Perhaps the excellent engineers who advise the Club Spares will then devise a cunning, bespoke rectangular seal to cure this problem. Or perhaps they are micro-climate-change deniers.