Yes, it is compatible with the 2019 27-inch iMac Retina 5K. The memory is DDR4 SO-DIMM at 2666 MHz, which matches the required specifications (original frequency 2667 MHz, tolerated), with a voltage of 1.20 V and 2 x 8 GB configuration. For best results, replace existing modules with identical kits in terms of speed and latency (CAS 18).