Acer AcerAltos 21000 System Manual page 75

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. Match the O on the cover with the O on
the retention mechanism first, then hooked them together and
then insert the OO side to the retention mechanism and make
sure they are clipped.
Removing an Intel Pentium II Xeon processor
Follow these steps to remove a Pentium II Xeon processor:
1. Remove the thermal and fan connector and then unclip the
retention mechanism cover.
System Boards 3-11