8GB (4GB x 2pcs) 1600Mhz
16GB (8Gb x 2pcs) 1600Mhz
32GB (8GB x 4pcs) 1600MHz
Timetec PC3-12800 DDR3L 1600MHz SO-DIMM 204 Pin CL11 SO-DIMM Memory Upgrade Kit for iMac 27" Late 2012, 2013, 2014 and early to Mid 2015 Retina 5K iMac 27-inch models.
Same for with iMac 21.5" model, late 2012, 2013 and 2014.