Nx Networks 3000 - Manual (Page 69)

3000 Series User Guide 55
Caution
If you have a slot in your 3000 Series that does not
have a module installed in it, be sure to install a blank
slot cover in each open slot to ensure proper airflow
inside the chassis.
7. Replace the cover.
Be careful to line up the motherboard’s screw holes with the chassis’
screw holes.