When did they switch buttons on 1966-68 Automatic console shifters from Potmetal to plastic?

[QuickBpBp]

Going through a few shifters I have and wanted to know when did they go from a Pot metal push button to a plastic push button? I have three floor shifters 2 are plastic and only one is potmetal. My '68 500 convertible is plastic so I know the changed occurred before then. So anyone know when they switched? Talking about this piece. Thanks in advance.

 

[AR67GTX]

My 67 GTX is plastic. I was just now re-installing it. Can’t be sure it’s original.

 

[R/T Boy 67]

My Feb. 67 built coronet's knob is plastic. It probably wasn't an overnight change.

 

[RemCharger]

66 metal, 67 up plastic, as far as I know

 

[turbine68rt]

I still have my original. I'll check and report back later.
4/68 build. Plastic knob.