Apple PC Compatibility Card (Installation) - Manual (Page 3)

You only need the plastic access port cover and metal retainer if you are
removing the Apple External Video Connector during the PC Compatibility
Card installation.
The Apple External Video Connector and the PC Compatibility Card cannot
be installed together into a 5400 series Macintosh computer. If your
computer has the video connector shown below, you will have to remove it
during the installation of the PC Compatibility Card.
Instructions for removing the Apple External Video Connector are included
in the installation instructions that follow.
If this connector is present, then an Apple External
Video Connector is installed in your computer.
Plastic access port cover Metal retainer
3 Installing the PC Compatibility Card in a 5400 Series Macintosh Computer