Before you can use a manual, you need the right one. Searching for "21 B6 E1 E2" verbatim will lead you to dead ends because that is not a product name.

While the specific manual depends on the model, most Intel desktop boards of this era follow a standardized assembly sequence:

Once you have this, search the Intel Download Center for that AA number to find the exact Product Guide. 2. Use Software (Digital Check)

: Press the Windows key, type "System Information," and look for the BaseBoard Product field. Common Boards with this Marking