A mirror has four pieces of hardware of the one appearing in the image, one at each corner.
Attempting to insert simultaneously four flat-head wood screws in these slots would likely be frustrating. Plus I'd rather leave myself some room for not getting the locations of the screws so absolutely accurate.
Is there some hardware that matches the one pictured? Would two screws be enough or, if four are present, four are necessary? Is this mirror meant to be hung as shown, or rotated 90 degrees? Are these pieces of hardware meant for somehow adding a cable, then hanging the mirror from that cable? Would the cable need to go around all four, perhaps loosely to distribute the load?