"So he has an old VESA mount where the mechanism in the middle did not fit. And he still simply screwed it together and it is held by pressing?"
On the outside you have this:
Or the Apple VESA Mount Adapter original equivalent (tab HF72EB in this link):
On the inside you have this:
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And you screw them together through the slot in the iMac case, which they are designed to do...
You need to make sure one or other of the sellers sells you the 9 screws.