What’s the “H” for on 915 heads?

Bennoel 10 said

I believe that's just a way to determine what casting mould that particular head was cast in. These are X heads with similar casting numbers and letters.

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Not trying to be disrespectful as I get the feeling I have offended some. I'm just trying to help/share what I believe. In the bottom picture of the X head I've shown is a # 09238. This is upside down in the pic. I believe this to be the actual casting date
09-sept.
23-day
8-year, 1968
Again I believe the letter that follows the casting # only tells what casting box it came from.