What’s the “H” for on 915 heads?

rumblefish360 said

I’ve never heard of a H or C head before. The top picture is a “J” head.

Click to expand...

The “H” and the “C” is a post script after the part number. Both a J heads.

Not trying to make a black swan here. I am just curious what the letter designation means. It may mean nothing and I’ve got my answer.