Buy Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM
iMac 27-Inch "Core i7" 4.0 (5K, Late 2015) Specs (Retina 5K, Late 2015, BTO/CTO, iMac17,1, A1419, 2834): EveryMac.com
Apple iMac (Retina 5K 27-inch Mid 2015) Serial Number Lookup - AppleSN.info
Buy Timetec 8GB Compatible for Apple Late 2015 iMac (27-inch w/Retina 5K Display) DDR3L 1867MHz / 1866MHz PC3L-14900 2Rx8 CL13 1.35V 204 Pin SODIMM Memory Module MAC RAM Upgrade for iMac 17,1
Install memory in an iMac - Apple Support
Install memory in an iMac - Apple Support
PSA: While Apple limits the new 27-inch iMac RAM to 32GB, you can double it to 64GB - 9to5Mac
Buy Timetec 32GB KIT(4x8GB) Compatible for Apple DDR3L 1600MHz PC3L-12800 CL11 for iMac (Mid 2011, Late 2012, Late 2013, Late 2014 Retina 5K, Mid 2015 Retina 5K) SODIMM Memory Module MAC RAM