I posted about Synology DDR4 RAM last June, and at that time, there were only five types of Synology DDR4 RAM available, out of which only there is only one ECC SO-DIMM which is the D4ECSO-2666-16G.
With the launch of the Synology 21 series, Synology added two more ECC SO-DIMM to their range of Synology DDR4 RAM. There are the D4ES01-4G and D4ES01-8G.