IBM F50 RS/6000 7025 - Manual (Page 215)

11. Install the backplane at the rear of bank D:
a. Align the top screw hole on the left side of the backplane with the top screw
hole on the left side of bank D, making sure the dimples on the backplane
line up with the holes on the frame of the server.
b. Secure the backplane with one screw.
Chapter 5. Installing Options 5-71