[dddd]

The anchor nut is M6x1 and is not 2mm thick, more like 3,5mm thick since such bolts are given a solid "reef" at installation.

I quickly found one such "thin" hex nut in my M6 NUTS drawer, it was from an old barrel adjuster of some sort I believe.

Absent any source of exact replacement parts, the challenge here is determining the dimensions of the needed nut. Perhaps you could start by probing the cavity with a 3mm hex key to estimate the needed thickness.

A bench grinder is very useful and would be the normal way to modify a standard hex nut into submission, but a belt-sander would also make very short work of this.

Even a vise and sharp file should be fully up to the duty of modifying a standard-thickness M6 hex nut in ten minutes or so.