What I thought. This head has inserts for unleaded fuel so maybe it was modified for tapered seat plugs. But I don't know why someone would do that. Either that or it wasn't used until 1970 production. But that seems hard to believe as well. There is another head that came with this one. I'll have to check the date on that one.
Jimmy it's been a while. Refresh my memory on the unleaded fuel inserts. I can't get the picture in my mind.
If it has hardened seat inserts, they were done in a machine shop somewhere - Chevrolet never used them. Unleaded gas wasn't mandated until 1975, and Chevrolet responded with electrically induction-hardened seat areas in the cast iron.