They were supplied to NZicon in crinkle finish. It seems to me that yours has been stripped off and just the original phosphate finish remains

The ejector screw is simplicity itself to make. You just need a philips headed bolt of the correct UNF thread and machine the short shoulder on it. Sterling bought them in as that and machined the shoulder.

NZ guns were sold to a UKicon dealer, originally to be sold on as serviceable weapons to other nations. But by then, SMG's were obsolete. So they remained unsold until they went out to the deact market where nobody really wanted them either!