If you pay close attention to the enlarged manifold in that thread, you'll see it's a later one with the reinforcing ribs. I believe they have more meat in them than the early ones. THere have also been some examples of factory manifolds with a 2 1/4" opening and on first thought, you think "Oh that's the Super 6 manifold, but that's not the case either, as people have found the smaller 1 7/8" outlet on original Super 6 engines. So I don't know what the rhyme or reason was or even if there was. What I'm more thinking is the differences in sizes was simply discrepancies in castings. Maybe someone knows more than that, but that's what I've come up with so far. If someone knows for sure what the larger outlet manifolds were actually used on, I'd be all ears. Since I've only seen ONE since I've been around cars, I'm sticking with my theory. lol