Same metal alloy to limit galvanic corrosion.

 

The new cage nuts I installed in the BlackSwan are GM OEM and are coated with a green corrosion resistant material.
the point of the replacement homemade nut is the size, it won’t spin because it wedges it the area it lives and will not be able to spin like it’s predecessor.
I recommend never seize and lots of it on the threads and shank of the bolt.

 

The threaded part looks to be the same thickness as the diameter of the bolt. That will help with stripping.
Can't tell the hardness just by looking at the nut.
There are a set of files available for a simple hardness test.

This stuff is by far the best antiseize and corrosion preventer, used primarily in the marine industry to combat salt and galvanic corrosion.

 

Metric has different hardness specs from ASE