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DDR4 2400MHz PC4-19200 for Apple iMac 2017 27-inch w/Retina 5K iMac 18
16GB Memory for Apple iMac Retina 5K, 27-inch (2020) DDR4 2666 MHz PC4-21300 RAM | eBay
Memory Upgrades for 2019-2020 iMac 27-Inch - Up to 128GB