Howdy Back Vonn:
To the best of our ability we can find no definitive date for the introduction of Hardened valve seats in 200/250 heads. The general jest of what we can find is that FoMoCo began using induction hardening on the exhaust seats only as early as 1973 and continued that practice until around the 1975-76 model year when both intake and exhaust seats were induction hardened.
With FoMoCo they never did any engineering changes all at once. When they had used up all of the old parts, new ones would show up. All of this is why we recommend finding a D7xx casting or later. By then all head have induction hardened seats. By then all heads have 1.75" intakes, a larger intake tract, a 1.75" carb hole as well as hardened valve seats. Unfortunately they also have a 62 cc chamber to reduce CR.
How to tell- It is next to impossible on a used head with just visual inspection. Occasionally, you might get a clue if the valve is wearing worse then the seat. The better way is when you machinist cuts into the seat while doing a valve job. An experienced machinist will know.
The induction hardening process is usually deep enough to still be effective after at least one valve job. Depending on what is done to the seat, maybe more.
Adios, David
