These are simply a plastic plate with 'walls' around the edge to enclose the toe end of the blades and stock rail. Cutouts are made to fit the rails and are shaped to hold the blades in mid travel. This was done since I reckoned if the blades were against one of the stock rails there was a small chance of paint getting drawn in by capillary action.
Small plastic handles are provided to allow the masks to be moved without handling the area that would be wet with paint (I was spraying the sides of the rails).