NOX decals were a result of Californina-mandated NOX devices that were required to be retrofitted to 1966-1970 cars and trucks registered in Calif. There were several different versions of NOX devices, but all were fundamentally designed to disable the vacuum advance mechanism as a way to reduce NOx emissions before the invention of the EGR valve. The most simplistic (and cheapest) NOX device was a set of rubber caps to plug the vacuum advance cannister and feed port and a sticker for the dashboard that said "Don't drive over 60 MPH". I am not making this up. The lack of vacuum advance could cause overheating at part throttle under load.
More sophisticated NOX devices actually incorporated a temp sensor that would re-enable the vacuum advance if the engine overheated. I have one of these in the barn someplace. It came on my 1966 442 that I bought in San Jose.
Starting with the 1970 model year, GM installed a system called Transmission Controlled Spark (TCS) that disabled vacuum advance unless the trans was in high gear. This essentially did the same thing as the NOX device. Of course, all of these devices went away when EGR showed up in 1972 on CA cars and in 1973 on 49-state cars.
As an example of how idiotic the emissions inspectors were at the time, I had a co-worker who had replaced the vacuum advance distributor in his Dodge van with a fully mechanical advance unit. The Calif emissions inspectors made him remove the mechanical advance distributor and reinstall a stock vacuum advance unit just so that he could install a NOX device to disable the vacuum advance.
