I'm sure you would be the first to agree that Ford has/will change and supercede any service part with whatever is handy and will work as a replacement. But that does not mean it is proper or correct for the original intended application. You've said many times how parts and numbers are changed and updated. The same, it seems, would apply to FE heads. Ten years later, if someone walked in to the dealer to buy a set of heads for their 406, they would get whatever was in production close enough to have the engine run. But that wouldn't necessarily be the correct valve or cumbustion chamber size original to the 406. I've got a list of about 15 different applications all with different variations of port, valve and combustion chamber sizes. I'm sure it's nowhere near complete. I can assure you if you were to try to sell me a '76 360 truck head for my '69 428 Mustang, I would be highly disappointed. Service parts don't necessarily have any bearing on original equipment. And that's all I was saying with my previous statement.