NVMe and CPU upgrades: 2019 vs 2017 27-inch iMacs?

What does the 5th Diagnostic LED on a 2017, 27-inch iMac do?

Antwoord op "NVME M.2 SSD with adapter"

Antwoord op "Can an SSD SATA drive be added to a 5K 2017 iMac that has a SSD blade?"

Antwoord op "Tear-down of new 2017 iMac 5k"

Antwoord op "iMac 2017 27" - i7-7700 possible?"

Antwoord op "New iMac 5k 2017 CPU is Upgradable?"

Antwoord op "Upgrading from 2TB Fusion Drive to NVMe/SSD Blade/PCIe SSD/M.2"

Antwoord op "The New iMac 27" 2017 model can upgrade ram and ssd?"

Antwoord op "What ssd PCIe works for iMac 27" 2017"