https://www.ebay.com/itm/1966-1967-...200635?hash=item4b6a50643b:g:fLcAAOSwwp9dcejM
The air vent knobs don’t get the discs, but all the other knobs do get them. If you look at the Dash knobs there is always glue residue inside where the disc was. If you look at the vent cables the plastic is smooth and new with no residue of the glue in them.
I made the mistake of putting them in my car years ago but realized that I never saw any other car with them in the vents knobs.
66-67 Coronets used the same air vent cable assemblies, and their dash knobs don’t look anything like the 66-67 Plymouth’s. So if they had the discs it would look very odd and attract attention to the mismatched knobs.