What’s the “H” for on 915 heads?

QQ- but long winded:

A few years ago I bought a pile of 340 parts. Part of the deal was a set of nicely painted and clean looking 3418915-H heads.
What the heck does the “H” stand for?
These are mildly ported heads, 2.02” and 1.60” configurations. Stamped 360 so I believe they are 71-72 vintage. I’m going to blow the dust off these and send them to machine shop for a quick going over. Maybe put them on a 340 build for a future project.
Thanks
Syleng1

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