MSI ATX BX7 User Manual page 9

Step 1: Insert the Retention Mechanism Attach Mount at the bottom
of the mainboard.
Step 2: Install the Retention Mechanism.
Look for the key on Slot 1, and match it with the Notch Key on the
Retention Mechanism for proper direction. Then, attach the
Retention Mechanism to the Retention Mechanism Attach Mount.
Use a Screwdriver to secure the Retention Mechanism.
Notch
Key
Key
SLOT1
Retention
Mechanism
Retention
Mechanism
Attach Mount
2-4