The shutters on that socket are indeed activated by the earth pin. They're a one-piece plastic moulding in a runner, with a bevelled tip for the earth pin. They simply get pushed out of the way against a spring when you insert the plug.
By the way, you seem to have been attempting to check continuity between contacts and terminals with the switch off. Not only were you probing the line pin, but that is almost certainly a dual pole switch, so even with the shutters open, you'd get nowhere like that on either side.
If you want to open it you'll have to drill out the rivets holding the earth bar on the back, the ones in the screw holes. Alternatively, just crack the whole plate off with a hammer or something.
E: Actually, that may be a socket which requires both live pins to open the shutter, not the earth. More prone to jamming.