Acer Altos 12000 Series User Manual page 76

3. Use the retention mechanism cover to secure the termination board by
pressing it down until it locks.
The retention mechanism cover only fits one way. Both shafts of the retention
mechanism and the retention mechanism cover have a O and a OO sign.
First, match the O on the cover with the O on the retention mechanism, then
hook them together, and finally, insert the OO side into the retention
mechanism and make sure they are clipped.
Altos 12000 Series User's Guide