When hardened valve seats production??

Thanks for all of the info and answers everyone. Good data.

I will add that I recall that unleaded fuel was mandated at all stations starting around '74; the addition of catalytcis followed that by a year or so. I can remember many small stations in '74 dropping leaded regular as they only had 2 pumps, and one had to be used for unleaded regular, and they saved the other one for premium.