Lenovo ThinkCentre M55 - Manual (Page 11)

Locating connectors on the rear of the computer
1 Power supply diagnostic LEDs 9 Serial connector (COM 2) (some models)
2 Voltage selection switch 10 USB connectors (8)
3 Power connector 11 Ethernet connector
4 Audio line out connector 12 PCI Express x16 adapter connector
5 Audio line in connector 13 PCI Express x1 adapter connector
6 VGA monitor connector 14 PCI adapter connector
7 Parallel connector 15 PCI adapter connector
8 Serial connector (COM 1)
Chapter 1. Locations 3