2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
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
512GB Apple PCIe Blade SSD Kit - iMac 21.5" A1418 Late 2013, Mid 2014, 2015 | eBay
SSD Upgrades for Apple iMac 27-Inch Late 2013 - 2019