The original stickers were short lived and often were used on 49 state vehicles being resold as used cars coming into the state. In the following decades there were cars that were not legal to be licensed in the state while others had to have a certain number of miles before they could be reregistered as used cars in the state.
The stickers were for NOX only and not issued as early as 68. You couldn't add the NOX kit to a car with a mechanical advance distributor. It restricted the vacuum line.
In practice many of us owners did use the NOX or $15 CarKit that one supplier provided to do just that. I installed a vacuum advance distributor and a close air cleaner to a 67 GT350 during one period since it was a better option than refitting (drilling) a valve into the nice COBRA intake.
The requirements, work arounds and requirements were changing all the time. If you could keep from going to a referee by doing some things it was often a better choice especially IMHO since the information on Fords BAR was often incorrect and misinterpreted overt and over
Even today systems are required thought they are not check or tested in an annual program. Do have my share of related manuals (for emission systems), training material and other books from Ford and the state. Use to get calls from owners on a fairly regular basis when they had issues and wanted to fallow the letter of the law and the state could use some help.