1TB Solid State Drive (SSD) PCIe for iMac 21.5-inch A1418 Retina(Late 2013 - Late 2015), iMac 27-inch A1419 Retina(Late 2013 - Late 2015), Mac mini ...
How to Add An SSD To The 27" iMac (Late 2015) - iFixit Repair Guide
SOLVED: Can I upgrade iMac (Late 2015) fusion drive to SSD? - iMac Intel 27" Retina 5k Display - iFixit
661-03562 Hard Drive 512GB (SSD) for iMac 27-inch Late 2015 A1419 MK462LL/A, MK472LL/A, MK482LL/A