Apple PC Compatibility Card (Installation) - Manual (Page 65)

9 Insert the adapter into the PCI slot on the main logic board. Make sure the metal fence on
the PC Compatibility Card engages the hook on the vertical plate and that the connectors
on the card protrude through the PCI access port opening.
If you meet resistance, pull the adapter out and try again. To see if the
adapter is properly connected, lift it gently. If it resists and stays in place,
it’s connected.
Vertical plate Card fence
The card is properly seated when the card’s metal
“fence” engages this hook on the vertical plate.
65 Installing the PC Compatibility Card in a Modular Desktop Macintosh Computer