Hi karl, as i said in your other thread it looked like a valve seat may have dropped as the damage is quite severe for any det etc. i would like to have a look at the valve seat grind and give you an opinion on why it failed if you want me to let us know.
it has been known if you have the head faced it will lower the amount of alloy to hold in the seat. a few cossies have dropped valves due to over skimming.