Black or grey phosphate would rust almost immediately in this location, black chromate would do much better but difficult to believe Chrysler would use a chrome screw and a black washer. I'm still of the belief a bright zinc washer would look best with the chrome screw as it left the factory. If it corroded a year or two later, not a factory issue. That close up Barry shows is really supportive of the idea but that screw and washer don't appear to be the same as shown by EL5 71, which look just like what I found on mine