You must have the 3rd type cover with both dimples and 2BA screw holes.

The gun would have originally had either the 2BA holes or the holes for the dimples. As Peter has said, if an armourer had the dimpled cover but not the screw type, or vice versa, they would modify the gun or cover to fit. If you have the holes in the gun for the dimples then you can put the screws away for some other use and just clip the cover on. If you just have the screw holes, then you need to screw it on.