PC4-21333 vs PC4-21300 RAM Apple support for adding extra RAM to 2019 iMac 27 5k retina specifies to add modules with these specs: Use Small Outline Dual Inline Memory Modules (SO-DIMM) that meet all of these criteria: PC4-21333 Unbuffered Nonparity 260-pin 2666MHz DDR4 SDRAM Online th...