Intel BOXD850MVSE Quick Reference Manual page 18

Follow these instructions to install an AGP card if it has a retention notch.
1 Observe the precautions in "Before You Begin" on 3.
2 Place the AGP card in the AGP connector.
3 Press down on the card until it is completely seated in the connector and
the card retention notch snaps into place below the RM.
4 Secure the card's metal bracket to the chassis back panel with a screw.