After a brief search I can only find this picture showing the location of the two screws that secure it to the backplate (one roughly central, one at the left hand side). I'm not familiar with that controller, but I expect you'll find that once you loosen these two screws the cover will open at the bottom and then disengage from lugs at the top. In my limited experience, slotted screws are more common for these fittings than cross point, so that's what I'd try first.
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