Original 512GB SSD MZ-JPU512T/0A6 For Apple iMac 21.5-inch A1418 Retina(Late 2013 – Late 2015), iMac 27-inch A1419 Retina(Late 2013 – Late 2015), Mac mini (Late 2014), Mac Pro (Late 2013), MacBook Air
NVME 512GB SSD For Apple iMac 21.5 A1418 EMC 2638 Late 2013 3 Years Warranty | eBay
2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
Amazon.com: OWC 1TB Aura Pro X2 NVMe SSD Upgrade Kit Compatible with Select 27" and 21.5" iMac Models (Late 2013 - Current) : Electronics
OWC SSD Upgrade Kits for 21.5-Inch iMac (2013 - 2019)
DIY Bundle - 1TB OWC 6G SSD and HDD Kit for 21.5-Inch iMac