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I own heads fitted by Norton Bracebridge St   in 1960  which are very good castings . I also own  heads fitted by the AMC factory  in the mid 1960's which were from a different Foundry. There is a huge difference in the finish and grain of the alloy. I had to spend a day filing off lumps and getting bead blasting done to make the later head passable . What more proof do I need other than the evidence of my own eyes ?.