When did Chrysler switch from chrome to plastic A/C vents?

Hi,My old 70 swinger had factory air with the chrome vents under the dash. I recently got another 70 swinger but this one has the later style plastic vents under the dash. Did Chrylser switch mid year to plastic vents? I know that by 71 they were all plastic vents.

There is a difference in how those plastic duct assemblies marry up to the main plenum. Not sure what year they changed. Not easily adapted.
The small rectangle adjustable louver sections are interchangeable throughout the years. So when the pot metal louvers became pitted/broken one could just swap the black plastic ones in. I had considered sending the black plastic ones along with a group of other parts going out for vacuum metalize chrome restore. I learned that air flow would take that type of chrome finish right back off again.

67-69 Chrome, rectangular rear opening
70-72 Plastic, same rear opening
73-76 Plastic, square rear opening
Yours was probably early production 70

Have a pic?