What’s the “H” for on 915 heads?

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