When hardened valve seats production??

2 Darts said
According to the 72 DartDemon Owner's manual, /6 engines were designed to run on unleaded fuel. (V-8s were not included). The 73 Dart Owner's manual stated that all engines were designed to run on unleaded fuel. I infer that these are the model years when hardened valve seats were installed on the various engine lines.

Unleaded gas was 75 as that is the first year of the catylatic converter. They could have brought the hardened seats in earlier.

My dad bought a 75 Cordoba brand new and it was the first car where it had the "unleaded fuel only" sticker below the gas tank door.