HP ProLiant ML350 User Manual page 76

Turn the spring-loaded locking pin until it locks into the chassis.
11.
Open the expansion board retainer.
12.
Remove the expansion board or the expansion slot cover for expansion slot 1
13.
expansion slot cover" on page 70). The PCI-X expansion cage is supported only by slot 1.
Install the included PCI Express conversion card into expansion slot 1.
14.
With the PCI-X expansion cage option, the additional PCI-X slots are recognized as slot 1 by the ROM.
Close the expansion board retainer.
15.
("Removing the
Hardware options installation 76