Bill/Joe
I want to say your both correct.
67 and some 68 received the heater delete prior to the mandate in Jan 68 and again in Dec 68.
I think Joe is referring to the plate for sale on Ebay since it is being sold as a 69 Heater Delete set, which you could get here in Hawaii and I see them on a lot of cars with orginal owners. I also have seen a few on rusted out shells which are no longer with those cars
I have also seen some other Chevy/Fords with the Delete plates in both 67/68 and earlier as well.