Dell D31M Owner's Manual page 122

17. PCIe x4 (Slot4)
18. PCI (Slot3)
19. PCIe x16 (Slot2)
20. PCIe x1 (Slot1)
21. System fan connector
22. Chassis Intrusion Detection connector
23. Type-C connector
24. Processor socket
The following images indicate the location of the system board and provide a visual representation of the installation procedure.
122 Removing and installing Field Replaceable Units (FRUs)