The area filled in on one side of the ex port in the OE heads is to facilitate more coolant capacity in the head. There is a corresponding mild bulge in the adjacent intake port.
Apparently that’s less of a concern to the aftermarket head designers. They’re going for the flow instead.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
