PHS-memory's two listings under this DDR4 ECC RDIMM family carry the same 64GB but land on different speed grades — one at the higher end of DDR4 and one considerably slower, so this is a case where reading the speed and CAS figures actually matters more than usual.
Each listing is 1×64GB, rated 2666 MT/s CL19. Registered ECC memory like this is built for server and workstation boards that specifically call for it, and the two speed bins here are not interchangeable in a system that expects a particular 2666 MT/s on its memory bus.
As RDIMM, DDR4 in this family needs a board built for registered ECC support; it will not run in a consumer desktop board designed for unbuffered non-ECC memory.