Don't know about the pressure relief butt...
I thought ONLY the 427's had the screw in freeze plugs. It could be one of the "side oiler" service blocks that were never drilled to be side oilers. I've heard some people drill the
oil passages out and then it "becomes" a "side oiler" after that.
Are you saying that some 390 or 406 blocks DID have screw in feeze plugs?