Locknuts and earthing locknuts are used in conjunction with a male threaded cable gland and often an IP washer to positively locate equipment in a thin walled enclosure.
They are offered in plated brass and stainless steel to match the cable gland / enclosure.
The specification of earthing locknuts is that one side includes sharp teeth that ensure the equipotential bonding once the locknut is tightened.
For further alternative materials see the Raxton locknuts.