The shrouded earth pin is indeed unusual. However, is there actually an earth wire in the lead? The power supply probably doesn't need one. Many such devices have a 'dummy' plastic earth pin (just to open the shutters in the socket).
However, the other end of the mains lead looks like a three-pin IEC. If the earth pin isn't connected, then a danger would exist if this means lead were to be used elsewhere for some other application, where an earth IS required.