Compaq 247320-003 - Deskpro 4000 - 5166 Model 2500/CDS Reference Manual page 150

3. Attach the 3.5-inch drive to the 5.25-inch bracket and
secure the drive to the bracket with the screws provided.
4. Install the guide screw into the front screw hole on the
right-hand side of the bracket.
Some options use M3 metric thread hardware. Extra guide
screws are provided on the front of the chassis, under the
front bezel. The Compaq-supplied metric screws are
black.