I can only speak to 70/71 model year E-body mirrors, but I assume others were the same.

Early in the 70 model year, both the driver and passenger mirrors had a date code. Then at some point in the Aug/Sept timeframe, the date was removed from the drivers mirror. The passenger mirrors still retained the date.

On original mirrors I've seen, most 70 mirrors have MONTH-DMI-YEAR format. In late 70 and throughout 71, the format changed to MONTH-JC-YEAR. Not sure what the DMI vs JC designated. Probably the manufacturer of the glass.

I have 2 sets of NOS 70 mirrors and they all follow the convention above.