Replace sink with pre-wall element

The sink is now in place. The locknut has been reinstalled and a regular nut with a washer has been screwed on. I hope it holds without a locknut on the back of the pre-wall installation, but if the plumber does it that way, I want to trust it... especially since it is not clear to me whether the manufacturer Walraven even intended the use of a locknut on the back of the pre-wall installation.

That matches the very pragmatic solution of the installer. Once you know how it can work, it's always quite simple, in this case a simple nut from the front. Very nice :)