Based on the model number sequence provided (21 B6 E1 E2), this corresponds to the Intel Desktop Board DZ77RE-75K (part of the Intel 7 Series chipset boards, specifically the Z77 Express Chipset).
Your vintage Intel board has years of life left—all it needs is the right manual. Intel Desktop Board 21 B6 E1 E2 Manual
The AA Number: Look for a label with a code starting with AA followed by a 6- or 9-digit number (e.g., AA E21088-204). Based on the model number sequence provided (
Intel Desktop Board 21 B6 E1 E2 Manual: Contents and Organization Intel Desktop Board 21 B6 E1 E2 Manual