Nor can I (and I don't recall having seen it before, for a switch), provided that a flylead ('earth wire') from backbox to faceplate is present.Well, not really. The faceplate is earthed by the earth wire. I cannot see the point of that metal bar.
I wonder if it's there in case there is no flylead, in which case it sort-of 'doubles' the chances of the faceplate screws providing an effective connection between backbox and faceplate?
Kind Regards, John