This is a 4GB DDR4 ECC memory module from Synology. The model number is D4ES01-4G, and it is compatible with the 21 series (RS1221RP+, RS1221+, DS1821+, DS1621+). It came standard with the DS1821+, but I am selling it because I upgraded to a larger capacity memory. It will be carefully wrapped in bubble wrap and shipped via Nekopos. Thank you.