M471A4G43MB1 Samsung 32GB DDR4 2666MHz (260Pin SODIMM, 1.2V) RAM
[M471A4G43MB1]

5 of 5 Stars Very fast delivery and product exactly as described. It is working fine on our iMac Retina 5K, 2019. We received these modules last week, removed the two 4GB and replaced with 4 of these Samsung 32GB modules. This worked great and are easy to install. These went in with no issues and the system recognized them right away and posted well.
By the way, if you are in the process of going deep down the of RAM technology complexities, then take note on these ECC RAM. The iMac Pro uses 288 Pin ECC RAM modules (same as servers), whereas the iMac uses standard desktop 260 Pin modules. I was a bit curious if 128GB of ram would work on our iMac. It's working extremely well (actually brilliantly) as I type this very review.