This is my first Dell server. It's an over-engineered piece of nonsense. Use regular hex head chassis screws, the kind you would use to hold a motherboard in or screw down an accessory card in a normal machine. Using no rails just tighten the screw directly into the 2nd and last screw hole in each side where the rails would go. Since the screws are the wrong size for rails, they will screw in just a bit then seize up leaving the gap required to slide the tape drive into the media cage. Don't forget to push down the lever on the side of the media cage yo insert or remove. By the way, you might want to take out the cage cover plate just below where you are installing the drive and bend down the grounding clips so they don't touch the circuitry on the bottom of the drive above. Good luck routing the drive cables around the fan housing nonsense.
Steven