Dell Vostro 420 (Late 2008) - Manual (Page 57)

Chassis Support Bracket
57
Replacing the Chassis Support Bracket
1 Holding the bracket at an angle, insert the tabs on one end of the bracket
into the slots in the chassis.
2 While holding the bracket slightly raised, slide any cables as appropriate
into the cable clip for storage.
3 Lower the other end of the bracket to engage the remaining tab.
4 Rotate the flange closed and replace and tighten the securing screw.