Apple Mac Pro (Early 2009) - Manual (Page 73)

Chapter 3 Making a Good Thing Better 73
10 Replace the PCI bracket under the brace and tighten the screws to secure the card.
11 Replace the side panel following the instructions on page 50.
NOTICE: If you remove a card and don’t install a replacement, place a port access
cover over the empty slot to keep foreign objects out of the enclosure. An uncovered
slot affects the airflow that cools the internal components and may cause damage.
NOTICE: Always replace the side panel after installing components. Your Mac Pro
doesn’t operate properly without the side panel in place.
Brace
PCI bracket