Dell OptiPlex GX280 - Manual (Page 312)

If the card is full-length, insert the end of the card into the card guide bracket as you lower the card toward its
connector on the system board. Insert the card firmly into the card connector on the system board.
1 bracket within slot
2 bracket caught outside of slot
3 fully seated card
4 not fully seated card
8. Before you lower the retention arm, ensure that:
The tops of all cards and filler brackets are flush with the alignment bar.
The notch in the top of the card or filler bracket fits around the alignment guide.
9. Press the retention arm into place, securing the card(s) in the computer.