Compaq Deskpro AP550 Reference Manual page 71

E-2 Security Lock Provisions
2. Insert the tang (tongue) of the narrow bracket (Bracket A) into the
slot on the chassis and slide the U-shaped backet (Bracket B)
between it and the system unit cover; then install the self-tapping
screw included in the cable lock it.
Due to differences between chassis, the slot may be located in
a different position than shown.
Installing brackets A and B