Used Aftermarket Aluminum heads.

[ken-r-mer]

So the flathead is asymmetric like most other engines of the time to minimize the "slap" at TDC. I know late model engines use offset in the piston wrist / connecting rod area to accomplish the same thing.

When I pull the head, I'll use a dial indicator to see what the back side of the seated valve is in relation to the deck height. I'd think cutting the valve seats can also affect the valve seat height.