What’s the “H” for on 915 heads?

Bennoel 10 said
The engine was assembled before the car was being assembled so no waiting on the assembly line.

I know the engine was assembled before it go to the car.
However, if you read what I said, you never would have wrote that.

Bennoel 10 said
In fact not even in the same plant. I still believe they had more than one casting mould and produced several in one pour. If there is a problem with a casting in a head they can identify which mould it came from and correct the problem. Just like auto recalls, they can say from this date, vin#, etc.

If the J was one casting plant and the U was another, this I don’t know. It would explain a few things though.