IBM Power System LC921 9006-12P Service Manual page 48

Figure 28. Replacing the PCIe riser screws
5. If you need to replace the PCIe riser card (A) or (B); replace the screws that secure the PCIe riser card
to the PCIe riser support (C) as shown in Figure 29 on page 35.
34 Power Systems: Servicing the IBM Power System LC921 (9006-12P)