Not well known, but most earlier performer and torker and Holley street dominator/strip dominator were not that much better than OEM. Most was just marketing. RPM, not that familiar with, air gap somewhat, warrior some what familiar. But on the low end aftermarket ... Marketing ideation was getting the customer to "believe" it was better and not so much effort to actually making it better.
Doing some of the restorations in the past and most being small block Chevy or Ford, some of the aftermarket of the era are an actual improvement over OEM, but your in discussion of mid sixties styles of manifold and the earlier four barrels. Once Q-Jet, Holley Square bore and Carter AFB came on scene 68 and later, the old stuff is fairly mute.
Oldsmobile as a manufacture seemed to have a better grasp of flow characteristics and efficiency than some others. Case on point is the exhaust manifold. The two in the center are larger and flow better than Chevy or Dodge which are more restrictive, but use less material.
Prior to the smog era mandatory changes, Olds seemed to have better OEM stuff when it came to heads and manifolds (opinion). After smog and EGR, any aftermarket manifold or head that reduces or corrects the issues of restriction, poor flow, and heated cross overs, would be an improvement (opinion).
Compare OEM 1970 to after market...,? With out flow bench numbers..., hard to say. But history is the earlier after market manifolds were not much different (or better) than OEM.
