(Intel’s internal assembly number) – e.g., E12345-678 . Searching that on Intel's site may yield the original support page.
Before downloading any firmware, drivers, or layout specifications, you must extract the . This is a small, white barcode sticker placed directly on the board. Follow these steps to locate it: intel desktop board 21 b6 e1 e2 specification link
I couldn't include external links here; to get the official spec sheet or support page, search for "Intel Desktop Board D21B6 specification" or the board product code with revision "E1" or "E2" on Intel's support site or the product brief PDF. (Intel’s internal assembly number) – e