Yes, it would have been installed on the assembly line. No, as we both agree it's not in the parts catalogues. Which is a little unusual; usually something like that is at least shown as "Not Serviced", but I guess not in this case.
Can it have been a test for different assembly lines for a period of time? Did they continue to use this plastic cover into the 70ies? Picture showing a 69 Roadrunner with the same plastic cover. Can it have been partnumbered later by Chrysler?