Upgrade the memory in your 27" iMac with Retina 5K Display (Late 2015) with a model ID of iMac17,1 using the 4GB DDR3 1867 MHz SO-DIMM Memory Module from OWC / Other World Computing. This single memory module features a capacity of 4GB, a clock speed of 1867 MHz, a bus speed of PC3-14900, and a 204-pin SO-DIMM form factor. It operates using 1.35V, has a CAS Latency of CL=11, a cycle time of 1.875 ns, does not support ECC technology, and is not registered. Additionally, it is fully compliant with JEDEC specifications and is RoHS certified.
Please bear in mind that this RAM will only work with the Apple 27" iMac with Retina 5K Display (Late 2015). These systems have a model ID of iMac17,1.