Sorry, I forgot: on the Apple page you can see that even slightly faster clocked ones can be used for these iMacs!

Compatible RAM Designation: 1333 MHz PC3-10600 DDR3 SO-DIMMs (204-pin)
Officially 4 GB per slot with a total of 16 GB. However, 8 GB per slot with a total of 32 GB have already been successfully tested.
Source: https://everymac.com/systems...

One of the following RAM should fit your Mac:
https://www.digitec.ch/de...

So I installed this Ram in my iMac 21.5" (mid 2011) in January 2019. Since then, my iMac has continued to work without any problems. You can also read about this at Apple under "Installing RAM in the iMac" and the link " https://support.apple.com/de-ch... "! Because I also replaced the internal hard drive with an SSD at the same time, work with it is pretty fast (Photoshop, Illustrtor, etc.). With my action, I kept an iMac that had been declared "dead" alive and saved it from the rubble heap. Unfortunately, far too much is thrown away today that can actually be repaired and used with little effort. I owe my "free work" for the repair to my two non-left hands and various videos on how to take an iMac apart. There are enough instructions on the internet and it is instructive anyway (of course not for all people equally...!). ..... so go ahead! Good luck and greetings from R.Bamert