The theory behind the sideways D shaped exhaust ports is that with regular round ports, some exhaust flow in the header can revert backwards into the cylinder and disrupt intake flow.
When Jim Comstock designed the FullAuto heads he incorporated D ports in the casting for that reason.
Pontiac successfully used D exhaust ports on their stock car racing engines way back in the 1950s. I believe that the more recent very high hp Corvette V8s also use the D exhaust port. My 1360 Terry Prince heads also have this feature.
For the Commando, I tried a poor man's version with some slugs fitted into the exhaust header. At 4 to 5000 rpm they made zero difference. It is possible that they might help or hinder at a higher rpm, but I cannot test that easily. I'm also principally interested in anything that maximizes the midrange, although now and then the top end is useful. This is especially true if I happen to be chasing a certain hopped up 750 Commando which has high compression, hot cam and a Comstock ported head with D exhaust ports ( done with weld buildup). When that is happening who needs midrange?
Glen