Back in the 70's when the Chrysler Direct Connection [now called Mopar Performance] people developed the W series of performance heads for the LA series of small block engines they found that, for a given cross sectional area, the D shape, with a flat floor flowed more exhaust gas than any other shape, & this shape was adopted for the exhaust ports in these heads.
I believe that some Chevrolet heads, particularly big block heads, copied this basic shape for some of their performance models.
[can anyone confirm]

The Datsun A series engine has a flat floor in the exhaust port too. Perhaps those clever little Datsun Engineers were much more clever than we have yet given them credit for.

EDIT
OK, just read Benny's post. It looks like more than one group has come up with the same result.