Asus P4B266-M - Manual (Page 26)

2-6 Chapter 2: Hardware information
3. Position the CPU above the socket such that its marked corner
matches the base of the socket lever.
4. Carefully insert the CPU into the socket until it fits in place.
The CPU fits only in one correct orientation. DO NOT force the CPU
into the socket to prevent bending the pins and damaging the CPU!
5. When the CPU is in place, press it firmly on the socket while you push
down the socket lever to secure the CPU. The lever clicks on the side
tab to indicate that it is locked.
Gold Mark