Question about Apple iMac Retina

These are DDR4-2400 Mhz. If you want to install 16s, Corsair Vengeance (2x, 16GB, DDR4-2666, SO-DIMM 260 pin) and Kingston KCP426SD8/16 (16GB, DDR4-RAM) would be an option.

Official specs: https://support.apple.com/de-de...

For example, this one would fit: Kingston KCP426SS8/8

There are also extra kits from OWC: https://www.4mac.de/arbeitsspe...
But off the top of my head I don't know any dealer who offers them in Switzerland.