MSI ATX BX13 User Manual page 15

Step 3: Install the Heat Sink Support Base.
Look for the 2 holes across Slot 1, and match it with the 2 Heat Sink
Support Base. Take note that one hole/base is bigger than the other.
Notch
Hole
Push the Heat Sink Support Base onto the mainboard, until you hear
a click sound. Check for a perfect fit.
Retention
Mechanism
2-10
PC-3742
Heat Sink
Support Base