I confess that I hadn't noticed that photo of the front, hence my previous comment ..On the contrary John ,the metal clips of the decorative, clip on front, connect directly to the metal switch plate,not any plastic . Take a look at the op's picture of the front of the switch plate.
What we can see certainly looks like plastic, but that earth terminal makes me wonder what's on the other side. ...
I'm pretty confused now, in trying to reconcile the pictures of the front and back. You are probably right. For what it's worth, my comment quoted above does indicate that I presumed the presence of the earth terminal probably indicated that there was something which needed to be earthed!The 2 square holes are where the springs connect the decorative front to the switch plate.
Kind Regards, John