IBM 5000 - Manual (Page 110)

Installing the Rack Model in a Rack Enclosure
b. From the rear of the rack enclosure, insert a long screw through the holes
in the cable-management arm mounting panel and cage nut; then, tighten
the screws.
Notes:
1) You can choose to wait to attach the cable-management arm to the
rack until after you have attached the other end of the
cable-management arm to the server.
2) To facilitate alignment of the cable-management arm, be sure to insert
the screws through the first and fourth holes, as shown.
Chapter 5. Installing a Server in a Rack Enclosure 101